Newtons Third Law of Motion

Definition …

Whenever an object excerts a force on a second object, the second object excerts an equal and opposite force on the first object.

Also known as …..

The law of action/reaction

What three things does this law say ?

1. Forces always come in pairs

2. Paired forces are equal in magnitude and opposite in direction

3. Paired forces act on different objects

What does it say ?

1. Forces always originate from other objects

2. Forces only occur when objects interact with each other

3. A single isolated force is an impossibility

4. You must have a minimum of two objects in order for forces to exist

_{Reacton Froce …..}

Originates in the inertia of the second object. When a force tries to change its motion, it resists and pushes back on the first object

What are the three examples mentioned in class …

Collison balls, Rocket, Swimmer

What is the example of the Collision Ball ?

The first ball swings down and hits the sencond ball. The force on the second ball than causes it to move and swing away from the first. But the second ball resists this change in motion and pushes back with an equal force on the first ball. The first ball is stopped by this reaction force. n

What is the example of Swimming ?

To swim forward the swimer uses his/her hand to push the water backwards. The water resists this change in motion and pushes swimer forward

What is the example of A rocket ?

To propel itself forward through space, a rocket pushes off of its exhaust gasses. By accelerating them out the back of the jet engine, they resist, pushing the rocket forward. Event hough the gas molecules have relatively little mass, the large change in motion they undergoe causes their small inertia to resists a lot

Cancellation…….

even though the paired forces of the 3rd law are equal in magnitude and opposite in direction, they generally do not add to zero and cancel out because they act on different objects. Each object feels only one of the two forces and thus expiriences a net force and accelerates

Change in motion….

Even thoiugh the two object exert equal forces on each other, each expiriences a different chang in motion as a result of that force because each has a different mass (Inertia).

What are the examples of changes in motion seen in class ?

Rifle/bullet, and Jumping

Example of Riffle/Bullet …..

When a rifle is fired, the rifle pushes the bullet forward and the bullet pushes the rifle backwards with equal forces, but the bullet has a much greater acceleration due to its low mass and inertia.

Jumping example …

When you jump upwards from the earth, the earth and you feel forces of the same magnitude, but you have a much greater change in motion. Still, according to the 2nd law, the earth must expirience some small acceleration thanks to your force on it.