Aim: How do we calculate work in situations involving kinetic friction?

Thought Problem 1: You are traveling along a freeway at 65 mi/h. Your car has kinetic energy. You now brake to a stop because of congestion in traffic. Where is the kinetic energy that your car once had? That is, what form does it now take, and where is this form of energy located. Work DONE BY FRICTION OR HEAT OR THERMAL ENERGY OR INTERNAL ENERGY

Restatement of the work-energy theorem Wnet=∆KE The work done by the net force is equal to the change in kinetic energy. If kinetic friction is present, we can say that Wnet=-Wfriction + ∑Wother Forces =∆KE Wnet =-fk∆x +∑Wother Forces

Internal Energy The increase in internal energy of the system is equal to the product of the friction force and the displacement of the block ∆Eint=fk ∆x

Thought Question 2 In many situations, friction forces tend to reduce the kinetic energy of an object; however, friction forces can sometimes increase an object’s kinetic energy. Describe a situation in which friction causes an increase in kinetic energy. A block is on top of another block that is moving

Problem 1 1. A block of mass 6 kg initially at rest is pulled to the right by a constant horizontal force with magnitude F =12 N. The coefficient of kinetic friction between the block and the surface is 0.15. Find the speed of the block after it has moved 3.00 m.

Problem 2 2. A sledge located with bricks has a total mass of 18 kg and is pulled at constant speed by a rope. The rope is inclined at 20 degrees above the horizontal and the cart moves a distance of 20 m on the horizontal surface. The coefficient of kinetic friction between the sledge and surface is 0.5. a) What is the tension in the rope? b) How much work is done on the sledge on the rope? c) What is the increase in internal energy in the sledge and the surface due to friction?