Newton’s 3rd LawAction and Reaction

Force PairsA force is always applied by a

secondary objectThe object to which the force is

applied also exerts a force back on the initial object

Whenever there is one force, there is a second force

Newton’s 3rd LawWhenever one object exerts a force on

a second object, the second object exerts an equal and opposite force back on the first

For every action, there is an equal and opposite reaction

The “action” and “reaction” forces act on different objects, this results in a different acceleration for both

Example Two carts, one with a mass of 3

kg, and the other with a mass of 1 kg are sitting end to end on a desk. One the “explodes” and exerts a force of 50N on the other, what is the acceleration of each?

TensionForce exerted on a lightweight

string, cord, or cable

Example Two blocks are attached by a

string; block A is on the left and has a mass of 20 kg, block B on the right and has a mass of 10 kg. Block A is pulled with a force of 60 N (W). Find the acceleration of the two blocks and the tension on the string between the two.

Example

An elevator uses a counterweight as shown. The mass of the counterweight is 1000kg and the mass of a loaded elevator is 1150kg. Calculate the acceleration of the elevator and the tension in the cable.

Example

A mover is lifting a piano up to a second-story apartment. He is using a rope looped over two pulleys as shown. What force must he exert on the rope to lift the 1600kg piano at a constant speed?

ExampleA small 50g mass hangs from a thin

string in a car and can swing like a pendulum as shown. What is the angle and force of tension on the string when the car a) accelerates forward at 1.20 m/s2

b) moves at a constant velocity of 90 km/hr