Laws of Motion and Energy

Chapter Six: Energy and Machines

• 6.1 Energy and Conservation of Energy

• 6.2 Work and Power

• 6.3 Simple Machines

Investigation 6B

• How do simple machines operate?

Force, Work and Machines

6.3 Simple Machines

• A machine is a device, like a bicycle, with moving parts that work together to accomplish a task.

What kinds of energy are needed to make this machine work?

6.3 Input and output• To understand how a machine works, think

about input and output. • The input includes everything you do to make

the machine work.• The output is what the machine does for you.

Input

Output

6.3 Input and output• Input can be forces, energy, or power

supplied to make a machine work.

• Output is the forces, energy, or power provided by the machine.

ForcesEnergyPower

ForcesEnergyPower

6.3 Simple Machines• A simple machine is an unpowered

mechanical device, such as a lever. • Some other simple machines are a wheel and

axle, ropes and pulleys, gears, and a ramp.

6.3 Input and output• With a lever, the input force (also called

the effort) is the force you apply.

• The output force is the force exerted on the load you are lifting.

Input

Output

6.3 Simple machines

• One person can easily lift an elephant with a properly designed system of ropes and pulleys.

6.3 Work and energy• The output work done by a simple machine can

never exceed the input work done on the machine.

• Friction always converts some of the input work to heat and wear, so the output work is always less than the input work.

6.3 Real machines and efficiency

• The efficiency of a machine is the ratio of work output to work input.

Can you calculate the efficiency of this machine?

6.3 Real machines and efficiency

• Efficiency is usually expressed in percent.• Because some friction is always present, 100%

efficient machines are impossible.• An important way to increase the efficiency of a

machine is to reduce friction. – Ball bearings and oil reduce rolling friction. – Slippery materials such as TeflonTM reduce sliding

friction. – Designing a car with a streamlined shape reduces air

friction.

6.3 Real machines and efficiency

• Cars are not very efficient at using the energy in gasoline.

• Only 13 % of the energy in a gallon of gas is transformed into output work.

6.3 Mechanical advantage and levers

• You can make a lever by balancing a board on a log.

• Pushing down on one end of the board lifts a load on the other end of the board.

6.3 Mechanical advantage and levers

• All levers include a stiff structure that rotates around a fixed point called the fulcrum.

• Levers are found in many common machines.

6.3 Mechanical advantage and levers

• Mechanical advantage is the ratio of output force divided by input force.

• The input and output forces are different if the fulcrum is not in the center.

This lever has a mechanical advantage of 3.

6.3 Ropes and pulleys

• Ropes and strings carry tension forces along their length.

6.3 Ropes and pulleys

• The mechanical advantage of a pulley system depends on the number of strands of rope directly supporting the load.

6.3 Ropes and pulleys

• There are 2 strands of rope supporting this load, so the load feels 2 times your input force.

6.3 Ropes and pulleys

• What is the mechanical advantage of a pulley with three strings?

6.3 Gears, ramps and screws

• Gears allow rotating speeds to change while power stays constant.

• The rule for how two gears turn depends on the numbers of teeth on each.

6.3 Gears, ramps and screws• A ramp allows you to

raise a heavy cart with less force than you would need to lift it straight up.

• Ramps reduce the input force by increasing the distance over which the input force acts.

6.3 Gears, ramps and screws

• A screw is a simple machine that turns rotating motion into linear motion.

• A screw works just like a ramp that curves as it gets higher.

Technology Connection

A Mighty Energizing Wind

• There is a new kind of farm that is unlike any other – it doesn’t produce food - it produces energy from wind.

Chapter Activity

• Rube Goldberg is well known for creating fun illustrations that show how many simple steps can work together to accomplish something.

• For this activity, you will design and build a multi-step device that will pop a balloon.

Pop Goes the Balloon