# efficiency

## calculating efficiency

Efficiency can be calculated with the equation: Output work Efficiency = Input work 100% Consider a machine that puts out 6000 joules of work. To produce that much work from the machine requires the user to put in 8000 joules of work. To find the efficiency of the machine, substitute these values into the equation for efficiency: 6000 J 100% = 75% 8000 J Q: Rani puts 7500 joules of work into pushing a box up a ramp, but only 6700 joules of work actually go into moving the box. The rest of the work overcomes friction between the box and the ramp. What is the efficiency of the ramp? A: The efficiency of the ramp is: 6700 J 100% = 90% 7500 J

## machine efficiency

Efficiency is the percent of work put into a machine by the user (input work) that becomes work done by the machine (output work). The output work is always less than the input work because some of the input work is used to overcome friction. Therefore, efficiency is always less than 100 percent. The closer to 100 percent a machines efficiency is, the better it is at reducing friction. Look at the ramp in the Figure 1.1. A ramp is a type of simple machine called an inclined plane. It is easier to push the heavy piece of furniture up the ramp to the truck than to lift it straight up off the ground, but pushing the furniture over the surface of the ramp creates a lot of friction. Some of the force applied to moving the furniture must be used to overcome the friction with the ramp. Q: Why would it be more efficient to use a dolly to roll the furniture up the ramp? A: There would be less friction to overcome if you used a dolly because of the wheels. So the efficiency of the ramp would be greater with the dolly.

## machines and friction

A dolly is a machine because it changes a force to make work easier. What is work? In physics, work is defined as the use of force to move an object over a distance. It is represented by the equation: Work = Force x Distance All machines make work easier, but they dont increase the amount of work that is done. You can never get more work out of a machine than you put into it. In fact, a machine always does less work on an object than the user does on the machine. Thats because a machine must use some of the work put into it to overcome friction. Friction is the force that opposes motion between any surfaces that are touching. All machines involve motion, so they all have friction. How much work is needed to overcome friction in a machine depends on the machines efficiency.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

all machines make work easier.

``````-->  a. true

b. false
``````

you can get more work out of a machine than you put into it.

``````a. true

-->  b. false
``````

the percent of the work put into a machine (input work) that is actually used to do work (output work) is a measure of the machines

``````a) power.

-->  b) efficiency.

d) none of the above
``````

which equation is used to calculate the efficiency of a machine?

``````a) efficiency = input distance/output distance x 100%

b) efficiency = input force/output force x 100%

c) efficiency = input work/output work x 100%

-->  d) none of the above
``````

## diagram questions

No diagram questions associated with this lesson