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Vocabulary

Work: the transfer of energy 

Negative work:  The attempt to slow and object down (to reduce it's kinetic energy)

Displacement: A vector that describes the difference between final and initial position 

Force: A push or a pull on an object

 Now the next set of pictures.  The first picture is of a rocket sled on a frictionless track, the rockets on the back have been fired.  The caption under the pictures says " at time equal zero the rockets are fired pushing the crate to the right (the positive x direction)".  The second picture shows the rocket sled 100 meters down the track, the rockets are still on.  The caption under the picture says "after 2 secs the sled has traveled 100m to the left (in the negative x direction)".  

The Question is asked:  "Based on the pictures do you expect the sled to be moving slower or faster?"    What do you think? 

 This is a little harder to figure out, what if the first picture had an added caption "the rocket sled has a velocity of 100 m/s to the left (the negative x direction)"?  Well I hope you understand that the rockets are being used to brake the moving sled, to slow it down. 

This in negative work, when the vectors for force and displacement are pointing in the opposite direction it produces negative work.