Going back to the sled example. We said that friction slows down the sled, it changes its kinetic energy. The situation is just that an outside (or external) force changes the kinetic energy. This happens if this outside force is also a net force. This should be clear from our study of Newton’s Second Law – a net force causes an acceleration and acceleration is change in velocity and therefore change in kinetic energy.
When you are pushing on the sled to overcome friction there are two outside forces, you pushing and friction, but there is no net force and therefore no change in kinetic energy.
Just to complicate things a new term is introduced here. The net force that changes the energy of the object is said to do work on the object. This is not anything new, just the name is new. Work changes the energy of an object. This is called the work energy theorem. In general this can be both potential and kinetic energy. You could for instance lift something to the top shelf. You are then doing work on the object because it gains potential energy.
Work can only be done if the object moves as a result of the force being applied. The sled for instance has to start moving when you are pushing it and then has to go faster and faster. The work done depends on the force and how far you pushing or lifting the object.
Just keep in mind that this is really not anything new. We are looking at change in motion under a different context.
Sample Problem:
A 10 kg block on wheels is initially traveling 2 m/s. A force of 2 N pushes the block for 10 m. How much kinetic energy does the block have after this interval?
