Physics Lecture 31 Instructor: John H. Hamilton

Lecture overview This week, Work and Energy Work – What work is – How is work calculated Power – Calculating power

Work

Notes on work For work to occur there must be motion Applying a force to something that doesn’t move produces no work For work to occur, the direction of the force must be along the same line as the motion of the object If the direction of the force is opposite the direction of motion, the work is negative The SI unit for work is the Joule (J) it is a N*m

Diagram showing work done on a body

Example 1 Calculate the work required to raise 10.0 kg 100 m

Example 2 A person pulls a 50 kg crate across a floor 40 m by constant force of 100 N that is 37° from the horizontal floor. The floor exerts a friction force of 50 N. determine the work done by each force on the crate and the net work done on the crate.

Example 2 space

Work performed when the force is not constant In the preceding examples, we calculated the work performed by a constant force. What if the force varies with distance Since work is the product of force and distance we can use the area under the curve method!

power

Example 3 Calculate the power for a 1400 kg car to go up a 10° hill at 80 km/hr if the resisting force (drag and friction) is 700 N

Example 3 space

Lecture review Work – Product of a force and the distance through which it works. Distance traveled and the force must be along the same line Power – The rate at which work is done