1. Meaning of work
A. You do work on an object when you exert a force on an object that causes the object to move some distance.
B. fig. 1, pg. 98
2. No work without motion
A. In order for you to do work on an object the object must move some distance as a result of your force.
B. In order to do work on an object the force you exert must be in teh same direction as the objects motion.
3. Calculating work
A. the amount of work done on an object can be determined by multiplying force times distance
work = force x distance
practic problems pg. 100
Ch 4 – 2 Mechanical Advantage and Efficiency
ideal mechanical advantage -
actual mechanical advantage -
1.What is a machine?
A. Something that makes work easier on you.
1. Changes the amount of force you exert
2. The distance over which you exert your force
3. Or the direction in which you exert your force
A. Input – the force you exert
B. Output – the force exerted by the machine
3. Multiplying force – read paragraph on page 103
4.Multiplying distance – read paragraph on pg. 104
5.Changing direction – read paragraph on pg. 104
A. Number of times a force exerted on a machine is multiplied by the machine.
mechanical advantage = output force
A. Compares the output work to the input work
B. Expressed (written) as a percent
C. Higher the percent – the more efficient the machine is.
A. Divide the output work by the input work and multiply the result by 100 percent
Efficiency = output work x 100%
Practice Problems (page 106)
1. You cut the lawn with a hand lawn mower. You do 250,000 J of work to move the mower. If the work done by the mower in cutting the lawn is 200,000 J, what is the efficiency of the lawn mower?
2. You do 1,500 J of work in using a hammer. The hammer does 825 J of work on a nail. What is the efficiency of the hammer?
3. Suppose you left your lawn mower outdoors all winter. It’s now rusty. Of your 250,000 J of work, only 100,000 go to cutting the lawn. What is the efficiency of the lawn mower now?
1. Calculating power
A. Power is calculated by dividing the amount of work done by the amount of time taken to do the work.
power = work/time
D. Since work is equal to force times distance you can rewrite the equation for power as follows:
power = force x distance/time
1. 1 watt = 1 J/s
2. Power and energy
A. Power is the rate at which energy is transferred from one object to another or converted from one form to another.
B. read pg. 152-153
practice problems pg. 151