Peoples Physics Concepts

http://www.ck12.org Chapter 7. Momentum Conservation

7.1 Momentum

• Describe what momentum is.


• Calculate the momentum of objects and use conservation of momentum to solve basic problems.

Students will learn what momentum is and how to calculate momentum of objects. In addition, students will learn
how to use conservation of momentum to solve basic problems.

Key Equations

p=mvMomentum is equal to the objects mass multiplied by its velocity

∑pinitial=∑p

final The total momentum does not change in closed systems

Example 1

A truck with mass 500 kg and originally carrying 200 kg of dirt is rolling forward with the transmission in neutral
and shooting out the dirt backwards at 2 m/s (so that the dirt is at relative speed of zero compared with the ground).
If the truck is originally moving at 2 m/s, how fast will it be moving after it has shot out all the dirt. You may ignore
the effects of friction.

Solution

To solve this problem we will apply conservation of momentum to the truck when it is full of dirt and when it has

dumped all the dirt.

mivi=mfvf start by setting the initial momentum equal to the final momentum

(mt+md)vi=mtvf substitute the mass of the truck plus the mass of the dirt in the truck at the initial and final states

vf=

(mt+md)vi

mt

solve for the final velocity

vf=

(500 kg+200 kg)∗2 m/s

500 kg

plug in the numerical values

vf= 2 .8 m/s