Chapter 14Work, Power and Simple

Machines• Do work, son!

14.1: Work and Power

• What is work?– the product of force and distance

• Question: How does a stationary object begin moving? (Recall...From Chapter 12)– When an unbalanced force acts on it.

• Work is done when a force acts on an object in the direction the object moves.

Is work being done?

Work Requires Motion

Question: Does a weight lifter do work on the barbell to lift it over his head?

Answer: yes, force is up and barbell moves up

Stationary Objects

• Question: Is the weight lifter doing work while he holds the barbell stationary over his head? 

ANSWER

• Answer: NO, the barbell is stationary

• For a force to do work on an object, some of the force must act in the same direction as the object moves.  If there is NO movement, NO work is done!!!

Work Depends on Direction

• The amount of work done on an object, if any, depends on the direction of the force and the direction of the movement.

• A force does not have to act entirely in the direction of movement to do work.

Is work being done?

Is work being done????

• The force acts upward and to the right.

• The suitcase only moves to the right.

• Any part of a force that does not act in the direction of motion does NO work on an object

Calculating Work

• Work = Force x Distance

• Units of Force and Distance– SI unit for force is newtons– SI unit for distance is meters

Unit of Work

• The SI unit for work is – newton-meter or the JOULE (J)

  

• When a force of 1 newton moves an object 1 meter in the direction of the force, 1 joule of work is done.

Practice Problem• Imagine the weight lifter. The weight lifter lifts a

1600 newton barbell over his head.  Assume the barbell is lifted to a height of 2.0 meters.  What is the work done?

• Work = Force x Distance• Work = 1600 N x 2.0 m• Work = 3200 N m = 3200 J

What is Power?

• Power: – the RATE of doing work

• Doing work at a faster rate requires more power.  To increase power, you can increase the amount of work done in a given time, or you can do a given amount of work in less time

Q: Does a person shoveling snow do work? Explain.

• Answer: YES, because the shovel is moving in the same direction as the force being applied

Q: Does a snow blower do work? Explain

Answer: YES, but because the snow blower does the work in less time it has more POWER!!!

Calculating Power

• Power = Work / Time – Work is in joules (J)– Time is in seconds (s)

• The SI unit for POWER is the watt (W) = one joule per second– Thus, a 40-watt light bulb requires 40 joules

each second that it is lit.

Practice Problem

• You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds.  How much power is used to lift   the box?

• Power = work / time • Where is work in the problem?• Can we calculate work?

– W = F x d

– W = 72 N x 1.0 m = 72J

• Now plug work into the formula for power• Power = 72J/ 2.0s• P = 36 Watts

James Watt and Horsepower

Horsepower

• Horsepower (hp): common unit for power.  One horsepower is equal to about 746 watts. 

• FYI...Interesting side note: Horsepower is literally

based on the power output of a very strong horse!!!