PhysicsLecture 31

Instructor: John H. Hamilton

Lecture overview

• This week, Work and Energy• Work– What work is– How is work calculated

• Power– Calculating power

Work

• Work is defined as the product of a force and the distance through which that force moves an object

• the lines mean parallel to• Can use components of vectors to get to

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!

0 2 4 6 8 10 120

100

200

300

400

500

600

Force vs. distance for a linear spring

distance (m)

Force (n)

power

• Power is the RATE at which work is performed. It is the work done divided by the time it took to do the work

• The unit for power is the Watt (W) and has units of J/s

• since work f*d• because v is d/t

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