Newton's laws of motion facts for kids

Original publication of the first two laws, in Latin

Isaac Newton (1642–1727), the father of the study of dynamics – the study of motion – developed three sets of laws that are believed to be true because the results of tests done by scientists agree with the laws he produced.

First law

• If a body is at rest it remains at rest or, if it is in motion, it moves with uniform velocity, until it is acted on by a resultant force.

A "uniform velocity" means that an object moves at a constant speed without changing direction (i.e. in a straight line). A "resultant force" means that the forces acting on the object are not balanced. In other words, the first law states two things:

1. A stationary object will only move if there is an unbalanced force acting on it.
2. A moving object will only change speed or direction if there is an unbalanced force acting on it.

The first part states that a table resting on the ground will not move unless pushed. Although gravity is acting on the table, pulling it down, there is a reaction force from the ground which is pushing it back. The forces acting on the table are balanced, so the object will not move.

The second part is harder to understand. A ball rolling along a flat surface will slow down and eventually come to a halt. But this is because of friction, which is a force that slows the ball down. A ball rolling down a slope is also affected by friction, but the force of gravity that causes it to move is stronger. In a place without the forces of friction, air resistance and gravity (e.g. in outer space) a moving object would keep on moving in a straight line if there were no force to slow it down or change its direction.

Second law

Isaac Newton (1643–1727), the physicist who formulated the laws

• The resultant force is equal to mass times acceleration.

This law provides the definition and calculation of force through mass and acceleration. Newton's second law says that acceleration is dependent on the forces acting upon an object and the mass of the object. Therefore if the force is increased, the acceleration is increased. And the more mass the object has, the acceleration decreases.

For example, weight is a force that we feel on Earth, caused by the gravity. Weight is calculated as

where m is the mass of the object and g is the local gravitational acceleration (not to be confused with G, the universal gravitational constant), roughly equal to 9.8 meters per second² (32 feet per second²) on Earth.

We can express Newton's second law in terms of momentum. The momentum p of a particle is defined as the product of its mass m and velocity v:

Newton's second law states that

• the resultant force acting on a particle equals the time rate of change of momentum of the particle

For a particle of fixed mass (constant m),

which is the F = ma equation above in vector form.

Third law

Newton's third law. The skaters' forces on each other are equal in magnitude, and in opposite directions

• For every action, there is an equal and opposite reaction. Or every action always reacts in the opposite direction.

This is best understood with billiard balls, where you can easily see the action/reaction pairs of forces. Likewise, when you kick a football, not only does the ball move, but you feel a force on your foot.

The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.

A variety of action-reaction force pairs are evident in nature. Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. Since forces result from mutual interactions, the water must also be pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.

Consider the motion of a car on the way to school. A car has wheels which spin forwards. As the wheels spin forwards, they grip the road and push the road backwards. Since forces result from mutual interactions, the road must also be pushing the wheels forward. The size of the force on the road equals the size of the force on the wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for cars to move along a roadway surface.

Related pages

• Classical mechanics

Images for kids

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  Newton's laws of motion, combined with his law of gravity, allow the prediction of how planets, moons, and other objects orbit through the Solar System, and they are a vital part of planning space travel. During the 1968 Apollo 8 mission, astronaut Bill Anders took this photo, Earthrise; on their way back to Earth, Anders remarked, "I think Isaac Newton is doing most of the driving right now."

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  In space near Earth, the deployed satellite seems to always travel away from the deployer. However, the satellite's orbit or path is actually being curved around Earth due to the Earth's gravity.

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  Rockets work by producing a strong reaction force downwards using its rocket engine. This pushes the rocket upwards, without regard to the ground or the atmosphere.

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  A bouncing ball photographed at 25 frames per second using a stroboscopic flash. In between bounces, the ball's height as a function of time is close to being a parabola, deviating from a parabolic arc because of air resistance, spin, and deformation into a non-spherical shape upon impact.

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  Two objects in uniform circular motion, orbiting around the barycenter (center of mass of both objects)

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  Rockets, like the Space Shuttle Atlantis, work by propelling matter in one direction to push the craft in the other. This means that the mass being pushed, the rocket and its remaining onboard fuel supply, is constantly changing.

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  The total center of mass of the forks, cork, and toothpick is on top of the pen's tip

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  Three double pendulums, initialized with almost exactly the same initial conditions, diverge over time.

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  Emmy Noether (1882–1935), who proved a celebrated theorem that relates symmetries and conservation laws, a key development in modern physics that is conveniently stated in the language of Lagrangian or Hamiltonian mechanics.

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  A simulation of a larger, but still microscopic, particle (in yellow) surrounded by a gas of smaller particles, illustrating Brownian motion.

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  Page 157 from Mechanism of the Heavens (1831), Mary Somerville's expanded version of the first two volumes of Laplace's Traité de mécanique céleste. Here, Somerville deduces the inverse-square law of gravity from Kepler's laws of planetary motion.

See also

In Spanish: Leyes de Newton para niños