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This text is meant to accompany class discussions. It is not everything there is to know about energy. It is meant as a  prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations (click here.)
 
Recoil

 

Recoil is occurs when momentum is exchanged and two or more bodies separate from each other. Recoil is backwards from a completely inelastic collision. In other words, what looks like one body becomes multiple bodies. In the animation above the spaceship (one body) explodes a heavy ball out of the back, (it becomes multiple bodies.) Mathematically it is modeled by...

In this case the ball and rocket are going in opposite directions. Since momentum is a vector, it has direction. This means that one of the velocities must be negative. If we adopt math class conventions, the velocity traveling right is positive and the velocity traveling left is negative. Therefore, in this example, the rocket's velocity is positive and the ball's velocity is negative.

Below is my YouTube video showing how to solve a "recoil" style problem using the law of conservation of momentum.

 

Math Example
  • Question
  • Solution
  • Video Solution

Question's Image

A truck is hauling a smaller car on its flat bed as shown. Somehow the car is released and rolls backwards off the truck at 4 m/s. What is the velocity of the truck after the car rolls off?

Question's Image

A truck is hauling a smaller car on its flat bed as shown. Somehow the car is released and rolls backwards off the truck at 4 m/s. What is the velocity of the truck after the car rolls off?

Solution

 
 

by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)