Work and Machines Chapter 5 Section 1

What is work? Work is the transfer of energy that occurs when a force makes an object move

The amount of work is dependent on a. the force exerted b. the distance over which the force is applied Work = Force X distance

Two conditions that must be met for work to occur An object MUST MOVE! Explain why this is true. The motion of the object must be in the same direction as the applied force on the object

How are work and energy related? 1. Energy is always transferred. . . a. from the object doing the work b. to the object on which the work is done. 2. Both work & energy are measured in Joules, the SI unit of work & energy 3. The unit Joule was named after an English physicist, James Prescott Joule. 4. One Joule is equal to a Newton-meter (N●m)

F D w Calculating Work Work equals force (in Newtons) times distance w= f x d F D w

Practice Problems The brakes on a bicycle apply 125 N of frictional force to the wheels as the bicycle travels 14.0 m. How much work have the brakes done on the bicycle? F = 125 N d = 14.0 m W = ? W = F x d W = (125 N)(14.0 m) W = 1,750 J

Practice Problems A hydraulic lift performs 12,500 J of work raising a car 0.5 m. How much force has the lift applied to the car? W = 12,500 J d = 0.5 m F = ? F = W / d F = 12,500 J / 0.5 m F = 25,000 J

POWER 1. The amount of work done in a certain amount of time (rate at which work is done). 2. Power is measured in Watts (W), named after the Scottish inventor James Watts.

Calculating Power Power equals work divided by time P=w/T Power is measure in watts (W)

Power Triangle P t w

For example The number of Watts marked on light bulbs tells you how much work the bulb does in a period of time - that is power. Do you see how work and power and related?

Practice Problems While rowing in a race, John does 3960 J of work on the oars in 60.0 s. What is his power output in watts? W = 3960 J P = W / t t = 60.0 s P = ? P = 3960 J / 60.0 s P = 66 W

Practice Problems While carrying a box of books for 20.0 s, your power output is 720 W. How much work have you done? P = 720 W W = Pt t = 20.0 s W = ? W = (720 J)(20.0 s) W = 14,400 J

Chapter 5 Section 2

Machines Transfer mechanical energy from one object to another Make work seem easier (cannot reduce the amount of work done) Can multiply force (wrench removing bolt), change the direction of a force (flagpole) or change the distance over which a force is applied (loading ramp)

Two types of forces are involved in machines: INPUT FORCE – force put in; also called the effort force OUTPUT FORCE – force out of machine; also called the resistance force

The same amount of work can be done by: 1. Applying a small amount of force over a long distance OR 2. By applying a large force over a short distance Increasing distance reduces the amount of force needed to do the work

Machines help move things that resist being moved Ex: Dolly used to move a heavy refrigerator

Mechanical Advantage Measures how much a machine multiplies force or distance MA>1: multiplies the effort force MA<1: increases distance or speed MA = output force = input distance input force output distance Has no units! (they cancel out)

An ideal machine with no friction would have the same input work and output work, however some energy transferred is change to heat due to friction

EFFICIENCY How much useful work a machine can do (expressed as %). Some work always overcomes friction, therefore some energy is always lost as heat! This means %eff < 100% !! % efficiency = work output x 100 work input % = WO / WI x 100

The efficiency of a machine is always less than 100 % Why? Friction plays a big role in the output of a machine