Unit 5 summary

 

The diagram above shows the puppies in a box at rest. There is no acceleration so the forces balance each other out.

 

In the diagram above, lets say the box is frictionless, so the puppies are moving at a constant velocity. Once again the forces are balancing each other out since there is no acceleration. 

Now in this drawing, the puppies are moving down a hill, which means they are experiencing friction. Because of this friction, they accelerate as they move down the hill, which means the forces are unbalanced.