Simple Machines & Work

Work: occurs when a force causes an object to move in the direction of the force.

Work= Force X Distance Force needed to move an object is equal to its weight. Increasing the amt. of force increases the amt. of work done. Increasing the distance increases the amt. of work done.

Joule (J): unit used to express work

A man applies a force of 500 N to push a truck 10 m down the street A man applies a force of 500 N to push a truck 10 m down the street. How much work does he do? F= 500 N X 10 m 5000 J

In which situation do you do more work? You lift a 75 N bowling ball 2 m off the floor. You lift two 50 N bowling balls 1 m off the floor.

Power: the rate at which work is done.

Power= Work / Time Power is increased when more work is done in a given amt of time. Power is increased when the time it takes to do a certain amt of work is decreased.

Watt: unit used to express power No matter how fast you can sand with sandpaper, an electric sander can do the same amount of work faster. Therefore, the electric sander has more power.

What is the power of a small motor that can do 400 J of work in 25 seconds? Power= 400 J / 25 s 16 W= 400 J / 25 s

Imagine changing a tire without a jack and the tire iron Imagine changing a tire without a jack and the tire iron. Would it be easy? No, you would have needed several people just to hold up the car! Sometimes you need a little help to do work. That’s where a machine comes in!

Machine: device that helps make work easier by changing the size or direction of a force

Work Input: work you do on a machine Work Output: work done by the machine

You need to understand that machines do not eliminate work You need to understand that machines do not eliminate work. They change the input work by size or direction, but not less work.

Do some machines work easier than others Do some machines work easier than others? Yes, because some machines can increase force more than others. A machine’s mechanical advantage tells you how many times the machine multiplies force. In other words, it compares the input force with the output force.

Mechanical advantage (MA) = Output Force / Input Force 10 What is the mechanical advantage if your input force is 50 N, but your output force is 500 N?

The less work a machine has to do to overcome friction, the more efficient it is. Mechanical efficiency is a comparison of a machine’s work output with the work input.

Mechanical Efficiency = (Work Output / Work Input) X 100 What is a machine’s mechanical efficiency if the work output is 400 N and its work input is 100 N? 400

Assignment: Quiz time. Take out a sheet of paper. Read a book. Work on any missing assignment.