Simple Machines

1. Simple Machines 7th grade science

2. Energy: conservation and transfer • 7.P.2.4 Explain how simple machines such as inclined planes, pulleys, levers and wheel and axles are used to create mechanical advantage and increase efficiency. • Remember that: the motion of an object can be described by its position, direction of motion and speed. • Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei, and the nature of a chemical. Energy is transferred in many ways.

3. Six simple machines that we will study • Inclined plane • Wedge • Screw • Lever • Wheel and Axle • Pulley

4. Three simple machines • Inclined plane, wedge, and screw • Inclined plane • Inclined planes have been in use for thousands of years. Whenever you need to raise something very heavy, it makes sense to use a ramp to help you. The Egyptians used inclined planes in constructing the pyramids. • Two basic concepts we need to understand the operation of an inclined plane: work and force. • Work is whenever you change energy from one form into another, we say that you are doing work. When you raise an object up, you increase its potential energy. This means you have done work.

5. Examples of inclined planes • Ramps, stairs, roads , slides… • Can you think of more…

6. Inclined plane • If you take a box that has a mass of 10 kg and raise it by a distance of 1 meter, you do an amount of work that is equal to approximately 100 joules. Remember that a joule is a unit of energy or unit of work. • Change in potential energy = work done = (mass) x (change in height) x (gravity) • Remember that “gravity” is 9.8 meters per second per second on Earth or approximately 10 meters.

7. An example • A 10kg mass is raised by 1 meter. The work done in this case is 100 joules. • The work depends on three things: the height difference, the mass, and the strength of gravity. 10 kg I meter 10 kg

8. Example, continued • On other planets, gravity has a different value. The 10 meters per second per second means this: if you drop an object, after one second, it will be falling at a speed of 10 meters per second. After two seconds, it will be falling at 20 meters per second. The speed increases by 10 meters per second, per second. • So we get this: 10 kg x 1 m x 10 m/s2 = 100 joules • Now, notice this: the change in energy depends only on the mass, gravity, and the change in height. Suppose we were to push this object up a ramp, as in the diagram on the next slide.

9. diagram • We push the object farther, certainly. In the diagram shown, the object is pushed about 2 meters. But the change in height is still 1 meter and since the work only depends on the change in height, the work done is still 100 J. • This is the key to the inclined plane: the work is still the same, but the work was done over a longer distance. Doing the same work over a longer distance means that you can do the work with less force. 10 kg 1 meter 10 kg

10. Work • Work done = force x distance over which force is applied. • In the above case, the work is done over twice as much distance: 2 meters instead of 1 meter. So we only need about half as much force to push the block up the ramp as we do to lift it. • And that’s it: that is why you use a ramp. You do the same work, but over a longer distance and so you need less force! • An inclined plane has a mechanical advantage that is determined by dividing the length of the incline by its height. • So the longer the incline, the greater the mechanical advantage. • Experiments: Changing Force with Changing Angle

11. Mechanical advantage • Some machines make work easier than others do because they can increase force more than other machines can. A machine’s mechanical advantage is the number of times the machine multiplies force. In other words, the mechanical advantage compares the input force with the output force. • The work done on a machine; the product of the input force and the distance through which the force is exerted is work input. • The work done by a machine; the product of the output force and the distance through which the force is exerted is work output. • Mechanical advantage (MA) = output force/input force • Figure 4 page 561

12. Mechanical efficiency • The less work a machine has to do to overcome friction, the more efficient the machine is. Mechanical efficiency is a comparison of a machine’s work output with the work input. • Mechanical efficiency = work output/work input x 100 • The 100 in this equation means that mechanical efficiency is expressed as a percentage. Mechanical efficiency tells you what percentage of the work input gets converted into work output.

13. The wedge • The wedge is one of the oldest of the simple machines. It is really a stone age tool that has been used for thousands of years for stripping bark from trees and splitting logs. • The wedge turns the vertical motion of the wedge into a horizontal force. The wedge, like other simple machines, changes the direction of a force. • The wedge works a bit like a ramp, or an inclined plane. The wedge needs to move a long way in order to split the wood just a small amount. This means that the force is amplified-a small force on the wedge turns into a large force on the wood. You can’t split a log with your bare hands, but the wedge amplifies the force so that the force of your body is all that is needed. • Experiment with the wedge

14. The screw • Most simple machines have one basic goal: to allow you to perform a task using less force than you would otherwise need. A simple machine allows you to supply much more force than you normally could. • Archimedes was the first person to do a serious investigation of the screw. In fact, he designed a screw for a particular purpose: raising water! turning a screw in a shaft whose end went into water at the end of the shaft, raised the water up. This had two benefits. • Changing the direction of the force. The shaft was rotated; this rotational force was changed, via the screw, into an upward force on the water. A rotational force is much easier to produce, by having an animal or a person push or pull a shaft as they walk around in a circle. The person or animal could produce a good deal of force this way. The screw turned this force into an upward force on the water.

15. The screw • 2) The screw allowed for great force amplification. The screw could lift a weight of water much greater than the weight the person or animal turning the screw could possibly lift. • The English unit for power, the horsepower, was figured by looking at how much work a horse could do during a day running an Archimedean screw by looking at how much water the horse could lift what distance. • Examples: propeller on a boat is a screw

16. Physics principles for the screw • There are two things that a screw does: • The screw converts the rotational motion of the screw into linear motion of the screw. • The screw provides an increase in available force. • As you turn the block that is attached to the screw, it moves about 1.4 meters. During this motion, the screw moves about 1 centimeter 0.01. • Now suppose a force pushes on the end of the screw while you turn the block. The work done in both cases is the same. Suppose you can apply a force of 100 N to the block. How much force will this produce on the end of the screw?

17. Calculate work done turning the screw • Work = force x distance • Work done turning screw = (100 N) x (1.40 m) = 140 J • This is equal to the work done by the force at the end of the screw. By equating the two work values, we can compute the force at the end of the screw. • 140 J = force x distance = (force) x 0.01 m) • And so we can calculate the force at the end of the screw: • Force on end of screw = (140 J)/(0.01 m) = 14,000 N • 14,000 N is the weight of 1400 kilograms: about 3000 pounds! By applying a modest force to the block, you can produce a force with the screw that is large enough to lift a small car. For example, changing a tire.