1. Kinetic Friction Friction is a non-conservative force that extracts energy from the system it acts on. The work done by friction is therefore changing the internal energy of the system.  = 180 o, so cos  = -1 If work is done to decrease the internal energy

2. A sports car accelerates from zero to 30 mph in 1.5 s. How long does it take for it to accelerate from zero to 60 mph, assuming the power of the engine to be independent of velocity and neglecting friction? 1. 2 s 2. 3 s 3. 4.5 s 4. 6 s 5. 9 s 6. 12 s

3. Example: A 20 kg crate is slid across the floor. It is pushed with a force of 500 N a distance of 20 m in 30 s. Friction acts on the crate with a force of 300 N. a) What is the work done by pushing? b) What is the work done by friction? c) What is the total work done? d) How much power was expended in each case? a) b) c) or d) 1 1 The net force is in the direction of the applied force.

4. CH 7: momentum and collisions

5. Momentum What is momentum? Resistance to change in the state of motion of a moving object. Inertia of a moving object. p – momentum [kg m/s]Momentum is a vector! What do we need in order to change the state of motion (i.e. change the velocity) of an object? A force is required for any acceleration (change in velocity) and therefore also required to change the momentum. or I – Impulse [kg m/s] Average Force Impulse is the force applied over a period of time to change the momentum. Apply a large force over a short time. Apply a small force over a long time.