Forces & Motion

Newton’s laws, gravity, friction and momentum — the forces that govern how everything in the universe moves.

A force is a push or pull that can change an object's motion.
Newton's first law states objects stay at rest or in motion unless acted upon.
Newton's second law relates force to mass times acceleration.
Newton's third law states every action has an equal and opposite reaction.
Gravity is the force that attracts objects toward one another.
Friction is a force that opposes motion between surfaces in contact.
Velocity describes both the speed and direction of a moving object.
Acceleration is the rate at which velocity changes over time.
Mass is the amount of matter contained in an object.
Weight is the force of gravity acting on an object's mass.
Momentum equals an object's mass multiplied by its velocity.
Inertia is the tendency of an object to resist changes in motion.
On Earth, objects in free fall accelerate at about nine point eight meters per second squared.
A lever is a simple machine that helps lift heavy loads.
Air resistance slows falling objects as they move through the atmosphere.
Pressure is the force applied per unit of area.
Isaac Newton published his three laws of motion in 1687.
Newton's laws appeared in his work the Philosophiae Naturalis Principia Mathematica.
Galileo Galilei studied the motion of falling bodies in the early seventeenth century.
Galileo argued that objects fall at the same rate regardless of their mass.
Aristotle wrongly believed heavier objects fall faster than lighter ones.
The SI unit of force is the newton, named after Isaac Newton.
One newton equals one kilogram meter per second squared.
Force is a vector quantity having both magnitude and direction.
Speed is a scalar quantity describing how fast an object moves.
Displacement is the straight-line distance and direction from start to finish.
Distance is a scalar quantity that ignores the direction of travel.
Acceleration occurs whenever an object speeds up, slows down, or changes direction.
The SI unit of acceleration is meters per second squared.
A net force of zero means an object's velocity stays constant.
Balanced forces produce no change in an object's state of motion.
Unbalanced forces cause an object to accelerate in some direction.
The net force is the vector sum of all forces on an object.
Equilibrium occurs when all forces acting on an object balance out.
Static equilibrium describes an object at rest with no net force.
Tension is the pulling force transmitted through a rope, string, or cable.
The normal force acts perpendicular to a surface in contact.
Applied force is a force exerted on an object by a person or thing.
Friction always acts opposite to the direction of relative motion.
Static friction prevents a stationary object from starting to move.
Kinetic friction acts on an object that is already sliding.
Static friction is usually greater than kinetic friction for the same surfaces.
The coefficient of friction is a number describing how rough two surfaces are.
Rolling friction is generally much smaller than sliding friction.
Lubricants like oil reduce friction between moving parts.
Drag is the resistive force exerted by a fluid on a moving object.
Terminal velocity is the constant speed reached when drag balances gravity.
A skydiver reaches terminal velocity when air resistance equals their weight.
Streamlining reduces drag by allowing fluids to flow smoothly around an object.
Free fall is motion under the influence of gravity alone.
In a vacuum, a feather and a hammer fall at the same rate.
Apollo 15 astronaut David Scott dropped a feather and hammer on the Moon.
The Moon's surface gravity is about one sixth that of Earth.
The acceleration due to gravity is often denoted by the letter g.
Weight equals mass multiplied by the acceleration due to gravity.
An object's mass stays the same everywhere, but its weight can change.
Newton's law of universal gravitation describes attraction between any two masses.
Gravitational force is proportional to the product of the two masses.
Gravitational force decreases with the square of the distance between objects.
The gravitational constant G appears in Newton's law of gravitation.
Earth's gravity keeps the Moon in orbit around the planet.
The Sun's gravity holds the planets in their orbits.
Tides are caused mainly by the Moon's gravitational pull on the oceans.
Johannes Kepler described the elliptical orbits of the planets.
Kepler's first law states planets move in ellipses with the Sun at one focus.
An object in uniform circular motion moves at constant speed along a circle.
Centripetal force points toward the center of a circular path.
Centripetal force is required to keep an object moving in a circle.
Without centripetal force, an object would move in a straight line.
A satellite stays in orbit because gravity provides its centripetal force.
The momentum of a system is conserved when no external force acts.
The law of conservation of momentum applies to collisions and explosions.
In an elastic collision, both momentum and kinetic energy are conserved.
In an inelastic collision, kinetic energy is not conserved.
In a perfectly inelastic collision, the objects stick together afterward.
Impulse equals the change in an object's momentum.
Impulse equals force multiplied by the time over which it acts.
The SI unit of momentum is the kilogram meter per second.
A larger mass at the same velocity carries greater momentum.
Newton originally framed his second law in terms of momentum change.
Work is done when a force moves an object through a distance.
The SI unit of work and energy is the joule.
One joule equals one newton acting over one meter.
Work equals force multiplied by displacement in the direction of the force.
No work is done if an object does not move.
Kinetic energy is the energy an object has due to its motion.
Kinetic energy equals one half the mass times velocity squared.
Potential energy is stored energy due to position or configuration.
Gravitational potential energy increases as an object is lifted higher.
Elastic potential energy is stored in stretched or compressed springs.
The law of conservation of energy states energy cannot be created or destroyed.
Mechanical energy is the sum of kinetic and potential energy.
Power is the rate at which work is done or energy is transferred.
The SI unit of power is the watt, named after James Watt.
One watt equals one joule of energy transferred per second.
Horsepower is a unit of power equal to about 746 watts.
A simple machine changes the direction or magnitude of a force.
The six classic simple machines were described by ancient Greek thinkers.
The inclined plane reduces the force needed to raise a load.
The wedge is a moving inclined plane used to split or cut.
The screw is an inclined plane wrapped around a cylinder.
The pulley uses a wheel and rope to change the direction of a force.
The wheel and axle reduces the effort needed to move loads.
Mechanical advantage is the ratio of output force to input force.
A lever has a load, an effort, and a pivot called the fulcrum.
A first-class lever has the fulcrum between the effort and the load.
A see-saw is a familiar example of a first-class lever.
Archimedes said he could move the Earth with a long enough lever.
A moment is the turning effect of a force about a pivot.
The moment of a force equals force times perpendicular distance from the pivot.
Torque is the rotational equivalent of force in physics.
The SI unit of torque is the newton meter.
The center of mass is the average position of an object's mass.
An object balances when its center of mass is above its base.
A lower center of mass makes an object more stable.
A spring stretches in proportion to the force applied to it.
Hooke's law states the extension of a spring is proportional to the load.
Robert Hooke described his law of elasticity in the seventeenth century.
The spring constant measures how stiff a spring is.
Pressure equals force divided by the area over which it acts.
The SI unit of pressure is the pascal, named after Blaise Pascal.
One pascal equals one newton per square meter.
Atmospheric pressure at sea level is about 101,325 pascals.
Pressure in a fluid increases with depth below the surface.
Pascal's principle states pressure applied to a fluid is transmitted equally.
Hydraulic systems use Pascal's principle to multiply forces.
Buoyancy is the upward force a fluid exerts on a submerged object.
Archimedes' principle relates buoyant force to displaced fluid weight.
An object floats when its weight equals the buoyant force on it.
An object sinks when its weight exceeds the buoyant force.
Density is an object's mass divided by its volume.
Objects less dense than water tend to float on it.
Galileo used inclined planes to slow and study accelerating motion.
A projectile follows a curved path called a parabola near Earth's surface.
Projectile motion combines horizontal and vertical motion independently.
The horizontal velocity of a projectile stays constant if drag is ignored.
A projectile's vertical motion is governed by the acceleration of gravity.
The range of a projectile is the horizontal distance it travels.
On level ground, a launch angle of 45 degrees gives the greatest range.
Two objects dropped together hit the ground at the same time without air.
A graph of position versus time shows how an object moves.
The slope of a position-time graph represents velocity.
The slope of a velocity-time graph represents acceleration.
The area under a velocity-time graph represents displacement.
Uniform motion means traveling equal distances in equal time intervals.
Average velocity is total displacement divided by total time taken.
Instantaneous velocity is the velocity at a single moment in time.
A reference frame is the viewpoint from which motion is measured.
Relative velocity describes the motion of one object as seen from another.
Newton's cradle demonstrates conservation of momentum and energy.
Recoil of a gun illustrates Newton's third law in action.
A rocket moves forward by expelling gas backward at high speed.
Rocket propulsion is a direct application of Newton's third law.
Friction between tires and road allows a car to accelerate and brake.
Seat belts use Newton's first law to restrain passengers in a crash.
Banked curves help vehicles turn by tilting the road surface inward.
Crumple zones extend collision time to reduce force on passengers.
The slope of a frictionless incline determines an object's acceleration.
Galileo's principle of inertia influenced Newton's first law of motion.
The equations of motion describe uniformly accelerated linear movement.
One kinematic equation states final velocity equals initial velocity plus acceleration times time.
A vector can be broken into horizontal and vertical components.
Adding vectors requires accounting for both magnitude and direction.
The resultant is the single vector equal to a sum of vectors.
Forces drawn as arrows show direction and relative magnitude.
A free-body diagram shows all forces acting on a single object.
Weightlessness in orbit results from continuous free fall around Earth.
Astronauts float in orbit because they fall with their spacecraft.
Centrifugal effect is the apparent outward push felt in a rotating frame.
A spinning ice skater speeds up by pulling their arms inward.
Angular momentum is conserved when no external torque acts.
Angular momentum depends on mass, shape, and rotational speed.
A gyroscope resists changes to its axis of rotation.
Moment of inertia measures an object's resistance to rotational acceleration.
Friction generates heat as surfaces rub against each other.
Brakes use friction to convert a vehicle's kinetic energy into heat.
Parachutes increase air resistance to slow a falling person safely.
Heavier objects need greater force to achieve the same acceleration.
The greater an object's mass, the greater its inertia.
Energy can change form but the total amount stays constant.
A swinging pendulum converts energy between kinetic and potential forms.
At the top of its swing, a pendulum has maximum potential energy.
At the bottom of its swing, a pendulum has maximum kinetic energy.
Period is the time for one complete cycle of an oscillation.
Frequency is the number of oscillations per second, measured in hertz.
The hertz is the SI unit of frequency.
A simple pendulum's period depends on its length and gravity.
Resonance occurs when a system is driven at its natural frequency.
Elastic materials return to their original shape after deforming.
Stress is the force applied per unit area within a material.
Strain is the fractional change in length caused by stress.
Beyond its elastic limit, a material does not return to its shape.
Newton's second law can be written as force equals mass times acceleration.
The standard value of g is about 9.81 meters per second squared.
An object moving at constant velocity has zero acceleration.
Mass is measured in kilograms in the International System of Units.
Doubling an object's speed quadruples its kinetic energy.
A force can change an object's speed, direction, or shape.
Galileo is often called the father of modern observational physics.
The horsepower unit was popularized by James Watt to compare engines.

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Forces & Motion

Newton’s laws, gravity, friction and momentum — the forces that govern how everything in the universe moves.

A force is a push or pull that can change an object's motion.
Newton's first law states objects stay at rest or in motion unless acted upon.
Newton's second law relates force to mass times acceleration.
Newton's third law states every action has an equal and opposite reaction.
Gravity is the force that attracts objects toward one another.
Friction is a force that opposes motion between surfaces in contact.
Velocity describes both the speed and direction of a moving object.
Acceleration is the rate at which velocity changes over time.
Mass is the amount of matter contained in an object.
Weight is the force of gravity acting on an object's mass.
Momentum equals an object's mass multiplied by its velocity.
Inertia is the tendency of an object to resist changes in motion.
On Earth, objects in free fall accelerate at about nine point eight meters per second squared.
A lever is a simple machine that helps lift heavy loads.
Air resistance slows falling objects as they move through the atmosphere.
Pressure is the force applied per unit of area.
Isaac Newton published his three laws of motion in 1687.
Newton's laws appeared in his work the Philosophiae Naturalis Principia Mathematica.
Galileo Galilei studied the motion of falling bodies in the early seventeenth century.
Galileo argued that objects fall at the same rate regardless of their mass.
Aristotle wrongly believed heavier objects fall faster than lighter ones.
The SI unit of force is the newton, named after Isaac Newton.
One newton equals one kilogram meter per second squared.
Force is a vector quantity having both magnitude and direction.
Speed is a scalar quantity describing how fast an object moves.
Displacement is the straight-line distance and direction from start to finish.
Distance is a scalar quantity that ignores the direction of travel.
Acceleration occurs whenever an object speeds up, slows down, or changes direction.
The SI unit of acceleration is meters per second squared.
A net force of zero means an object's velocity stays constant.
Balanced forces produce no change in an object's state of motion.
Unbalanced forces cause an object to accelerate in some direction.
The net force is the vector sum of all forces on an object.
Equilibrium occurs when all forces acting on an object balance out.
Static equilibrium describes an object at rest with no net force.
Tension is the pulling force transmitted through a rope, string, or cable.
The normal force acts perpendicular to a surface in contact.
Applied force is a force exerted on an object by a person or thing.
Friction always acts opposite to the direction of relative motion.
Static friction prevents a stationary object from starting to move.
Kinetic friction acts on an object that is already sliding.
Static friction is usually greater than kinetic friction for the same surfaces.
The coefficient of friction is a number describing how rough two surfaces are.
Rolling friction is generally much smaller than sliding friction.
Lubricants like oil reduce friction between moving parts.
Drag is the resistive force exerted by a fluid on a moving object.
Terminal velocity is the constant speed reached when drag balances gravity.
A skydiver reaches terminal velocity when air resistance equals their weight.
Streamlining reduces drag by allowing fluids to flow smoothly around an object.
Free fall is motion under the influence of gravity alone.
In a vacuum, a feather and a hammer fall at the same rate.
Apollo 15 astronaut David Scott dropped a feather and hammer on the Moon.
The Moon's surface gravity is about one sixth that of Earth.
The acceleration due to gravity is often denoted by the letter g.
Weight equals mass multiplied by the acceleration due to gravity.
An object's mass stays the same everywhere, but its weight can change.
Newton's law of universal gravitation describes attraction between any two masses.
Gravitational force is proportional to the product of the two masses.
Gravitational force decreases with the square of the distance between objects.
The gravitational constant G appears in Newton's law of gravitation.
Earth's gravity keeps the Moon in orbit around the planet.
The Sun's gravity holds the planets in their orbits.
Tides are caused mainly by the Moon's gravitational pull on the oceans.
Johannes Kepler described the elliptical orbits of the planets.
Kepler's first law states planets move in ellipses with the Sun at one focus.
An object in uniform circular motion moves at constant speed along a circle.
Centripetal force points toward the center of a circular path.
Centripetal force is required to keep an object moving in a circle.
Without centripetal force, an object would move in a straight line.
A satellite stays in orbit because gravity provides its centripetal force.
The momentum of a system is conserved when no external force acts.
The law of conservation of momentum applies to collisions and explosions.
In an elastic collision, both momentum and kinetic energy are conserved.
In an inelastic collision, kinetic energy is not conserved.
In a perfectly inelastic collision, the objects stick together afterward.
Impulse equals the change in an object's momentum.
Impulse equals force multiplied by the time over which it acts.
The SI unit of momentum is the kilogram meter per second.
A larger mass at the same velocity carries greater momentum.
Newton originally framed his second law in terms of momentum change.
Work is done when a force moves an object through a distance.
The SI unit of work and energy is the joule.
One joule equals one newton acting over one meter.
Work equals force multiplied by displacement in the direction of the force.
No work is done if an object does not move.
Kinetic energy is the energy an object has due to its motion.
Kinetic energy equals one half the mass times velocity squared.
Potential energy is stored energy due to position or configuration.
Gravitational potential energy increases as an object is lifted higher.
Elastic potential energy is stored in stretched or compressed springs.
The law of conservation of energy states energy cannot be created or destroyed.
Mechanical energy is the sum of kinetic and potential energy.
Power is the rate at which work is done or energy is transferred.
The SI unit of power is the watt, named after James Watt.
One watt equals one joule of energy transferred per second.
Horsepower is a unit of power equal to about 746 watts.
A simple machine changes the direction or magnitude of a force.
The six classic simple machines were described by ancient Greek thinkers.
The inclined plane reduces the force needed to raise a load.
The wedge is a moving inclined plane used to split or cut.
The screw is an inclined plane wrapped around a cylinder.
The pulley uses a wheel and rope to change the direction of a force.
The wheel and axle reduces the effort needed to move loads.
Mechanical advantage is the ratio of output force to input force.
A lever has a load, an effort, and a pivot called the fulcrum.
A first-class lever has the fulcrum between the effort and the load.
A see-saw is a familiar example of a first-class lever.
Archimedes said he could move the Earth with a long enough lever.
A moment is the turning effect of a force about a pivot.
The moment of a force equals force times perpendicular distance from the pivot.
Torque is the rotational equivalent of force in physics.
The SI unit of torque is the newton meter.
The center of mass is the average position of an object's mass.
An object balances when its center of mass is above its base.
A lower center of mass makes an object more stable.
A spring stretches in proportion to the force applied to it.
Hooke's law states the extension of a spring is proportional to the load.
Robert Hooke described his law of elasticity in the seventeenth century.
The spring constant measures how stiff a spring is.
Pressure equals force divided by the area over which it acts.
The SI unit of pressure is the pascal, named after Blaise Pascal.
One pascal equals one newton per square meter.
Atmospheric pressure at sea level is about 101,325 pascals.
Pressure in a fluid increases with depth below the surface.
Pascal's principle states pressure applied to a fluid is transmitted equally.
Hydraulic systems use Pascal's principle to multiply forces.
Buoyancy is the upward force a fluid exerts on a submerged object.
Archimedes' principle relates buoyant force to displaced fluid weight.
An object floats when its weight equals the buoyant force on it.
An object sinks when its weight exceeds the buoyant force.
Density is an object's mass divided by its volume.
Objects less dense than water tend to float on it.
Galileo used inclined planes to slow and study accelerating motion.
A projectile follows a curved path called a parabola near Earth's surface.
Projectile motion combines horizontal and vertical motion independently.
The horizontal velocity of a projectile stays constant if drag is ignored.
A projectile's vertical motion is governed by the acceleration of gravity.
The range of a projectile is the horizontal distance it travels.
On level ground, a launch angle of 45 degrees gives the greatest range.
Two objects dropped together hit the ground at the same time without air.
A graph of position versus time shows how an object moves.
The slope of a position-time graph represents velocity.
The slope of a velocity-time graph represents acceleration.
The area under a velocity-time graph represents displacement.
Uniform motion means traveling equal distances in equal time intervals.
Average velocity is total displacement divided by total time taken.
Instantaneous velocity is the velocity at a single moment in time.
A reference frame is the viewpoint from which motion is measured.
Relative velocity describes the motion of one object as seen from another.
Newton's cradle demonstrates conservation of momentum and energy.
Recoil of a gun illustrates Newton's third law in action.
A rocket moves forward by expelling gas backward at high speed.
Rocket propulsion is a direct application of Newton's third law.
Friction between tires and road allows a car to accelerate and brake.
Seat belts use Newton's first law to restrain passengers in a crash.
Banked curves help vehicles turn by tilting the road surface inward.
Crumple zones extend collision time to reduce force on passengers.
The slope of a frictionless incline determines an object's acceleration.
Galileo's principle of inertia influenced Newton's first law of motion.
The equations of motion describe uniformly accelerated linear movement.
One kinematic equation states final velocity equals initial velocity plus acceleration times time.
A vector can be broken into horizontal and vertical components.
Adding vectors requires accounting for both magnitude and direction.
The resultant is the single vector equal to a sum of vectors.
Forces drawn as arrows show direction and relative magnitude.
A free-body diagram shows all forces acting on a single object.
Weightlessness in orbit results from continuous free fall around Earth.
Astronauts float in orbit because they fall with their spacecraft.
Centrifugal effect is the apparent outward push felt in a rotating frame.
A spinning ice skater speeds up by pulling their arms inward.
Angular momentum is conserved when no external torque acts.
Angular momentum depends on mass, shape, and rotational speed.
A gyroscope resists changes to its axis of rotation.
Moment of inertia measures an object's resistance to rotational acceleration.
Friction generates heat as surfaces rub against each other.
Brakes use friction to convert a vehicle's kinetic energy into heat.
Parachutes increase air resistance to slow a falling person safely.
Heavier objects need greater force to achieve the same acceleration.
The greater an object's mass, the greater its inertia.
Energy can change form but the total amount stays constant.
A swinging pendulum converts energy between kinetic and potential forms.
At the top of its swing, a pendulum has maximum potential energy.
At the bottom of its swing, a pendulum has maximum kinetic energy.
Period is the time for one complete cycle of an oscillation.
Frequency is the number of oscillations per second, measured in hertz.
The hertz is the SI unit of frequency.
A simple pendulum's period depends on its length and gravity.
Resonance occurs when a system is driven at its natural frequency.
Elastic materials return to their original shape after deforming.
Stress is the force applied per unit area within a material.
Strain is the fractional change in length caused by stress.
Beyond its elastic limit, a material does not return to its shape.
Newton's second law can be written as force equals mass times acceleration.
The standard value of g is about 9.81 meters per second squared.
An object moving at constant velocity has zero acceleration.
Mass is measured in kilograms in the International System of Units.
Doubling an object's speed quadruples its kinetic energy.
A force can change an object's speed, direction, or shape.
Galileo is often called the father of modern observational physics.
The horsepower unit was popularized by James Watt to compare engines.

Learn this lesson, not just read it

Load these facts straight into the free Class Coupon Speed Learning app — they become fast, playful study activities with audio and translation in 100+ languages.

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