4.40. Efficiency http://www.ck12.org
4.40 Efficiency
- Explain why all machines must overcome friction.
- Define the efficiency of a machine.
- Show how to calculate the efficiency of a machine.
This man is pushing heavy boxes on a dolly. If he were to push the boxes across the ground without the dolly,
it would take a lot more force because he would have to overcome the sliding friction between the boxes and the
ground. Using a dolly reduces the friction because rolling friction between the wheels and the ground is much less
than sliding friction.
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 don’t 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. That’s 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 machine’s efficiency.