Work Power and Machines Physical Science Chapter 13

Work, Power, and Machines Physical Science Chapter 13 Section 1

Work • Occurs when a force causes an object to change its motion or position. • Calculated by multiplying the force by the distance over which the force is applied. • Work = force x distance • W = FD

Work • This weightlifter is holding a barbell over his head. Is he doing any work on the barbell?

Work • Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift if he lifts her 2 m and exerts an average force of 190 N? – Force, f = 190 N – Distance, d = 2 m – Work, W = ? J • W = 190 N x 2 m = 380 J

Work Practice #1 • A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do on the girder?

Work Practice #2 • An apple weighing 1 N falls a distance of 1 m. How much work is done on the apple by the force of gravity?

Work Practice #3 • A bicycle’s brakes apply 125 N of frictional force to the wheels as the bike move 14 m. How much work do the brakes do?

Work Practice #4 • A mechanic uses a hydraulic lift to raise a 1200 kg car 0. 5 m off the ground. How much work does the lift do on the car?

Work Practice #5 • A car breaks down 2. 1 m from the shoulder of the road. 1990 N of force is used to push the car off the road. How much work has been done on the car?

Work Practice #6 • Pulling a boat forward into a docking slip requires 1570 J of work. The boat must be pulled a total distance of 5. 3 m. What is the force with which the boat is pulled?

Work Practice #7 • A box with a mass of 3. 2 kg is pushed 0. 667 m across a floor with an acceleration of 3. 2 m/s 2. How much work is done on the box?

Power • The rate at which work is done. – Power = work/time – P = W/t

Power • Lifting an elevator m takes 100000 J. If doing so takes 20 s, what is the average power of the elevator during the process? – Work, w = 100000 J – Time, t = 20 s – Power, P = ? W – P = 100000 J/20 s = 5000 W (watts)

Power Practice #1 • While rowing across the lake during a race, John does 3960 J of work on the oars in 60 s. What is his power output in watts?

Power Practice #2 • Anna walks up the stairs on her way to class. She weights 565 N, and the stairs go up 3. 25 m. • If Anna climbs the stairs in 12. 6 s, what is her power output? • What is her power output if she climbs the stairs in 10. 5 s?

Machines • Machines change the size and/or direction of forces. • A mechanical advantage is a quantity that expresses how much a machine multiplies force or distance. • Mechanical advantage = output force/input force = input distance/output distance

Mechanical Advantage • Calculate the mechanical advantage of a ramp that is 5 m long and 1. 5 m high. • Input distance = 5 m • Output distance = 1. 5 m • Mechanical advantage = ? • Mechanical advantage = input distance / output distance = 5 m/ 1. 5 m = 3. 3

Mechanical Advantage Practice #1 • Find the mechanical advantage of a ramp that is 6 m long and 1. 5 m tall.

Mechanical Advantage Practice #2 • Alex pulls on the handle of a claw hammer with a force of 15 N. If the hammer has a mechanical advantage of 5. 2, how much force is exerted on the nail in the claw?