Chapter 7

Momentum and Collisions

Linear Momentum


Momentum is the mass times velocity.

SI unit:


Example 7-1

(a) A 1180 kg car drives along a city street at 30.0 mph (13.4 m/s). What is the magnitude of the car's momentum?



(b) A major league pitcher can give a 0.142 kg baseball a speed of 101 mph (45.1 m/s). Find the magnitude of the baseball's momentum?



Example 7-2

The figure shows a 2.0 kg toy race car before and after taking a turn on a track. Its speed is 0.50 m/s before the turn and 0.40 m/s after the turn. What is the change in the linear momentum of the car due to the turn?





Impulse and Momentum Change

The Law of Conservation of Momentum

If no net external force acts on objects in a system, the total linear momentum of the system cannot change.

Newton's Second Law was written in momentum form at first.

Newton's Second Law

The time rate change of the momentum of a particle is equal to the net force acting on the particle and is in the direction of that force.

Example 7-3

The figure gives the linear momentum versus time for a particle moving along an axis. A force directed along the axis acts on the particle. (a) Rank the four regions indicated according to the magnitude of the force, greatest first. (b) In which region is the particle slowing?



Example 7-4

At a city park, a person throws some bread into a duck pond. Two 4.00 kg ducks and a 9.00 kg goose paddle rapidly toward the bread, as in the figure. If the ducks swim at 1.10 m/s, and the goose swims with a speed of 1.30 m/s, find the magnitude and direction of the total momentum of the three birds.