Work and Simple Machines

S8P3. Students will investigate relationship between force, mass, and the motion of objects.

a. Determine the relationship between velocity and acceleration.

b. Demonstrate the effect of balanced and unbalanced forces on an object in terms of gravity, inertia, and friction.

c. Demonstrate the effect of simple machines (lever, inclined plane, pulley, wedge, screw, and wheel and axle)

on work.

Essential Questions

•When is work done on an object?•How do you determine the work done on an object?•What is the relationship between power and work?

WARM-UP:

Work – p.406-407

Work is done on an object when the

object moves in the same direction in which the force is

exerted.

Why is this server in a restaurant NOT doing any work?

Calculating Work – p.408-409

The amount of work done on an object can be determined

by multiplying force times distance.

Is work being done?

• Work = Force x Distance• Work = 5000 N x 200 m• Work = 1,000,000 Joules!

Power – p.409-411

Power is the rate at which work is done.

It is equal to the amount of work

done on an object in a unit of time.

WATT’s the unit of power?!?!

How Machines Do Work– p.412-415

A MACHINE is a device that allows you to do work one of three ways that is easier:

1. It can change the amount of force that you exert

2. It can change the distance over which you exert your force

3. It can change the direction in which you exert your force

Input and Output forces – p.413

Input force is the force that YOU exert on the machine, causing it to move a certain distance. (Ex: You exerted

a force on a shovel)Output force is the force that the MACHINE exerts

over another distance. (Ex: the shovel’s force on the

dirt)

Input and Output work– p.413

Input work is the input force times the input

distanceOutput work is the output

force times the output distance

When you use a machine, the amount of output work can never be greater than the amount of input work.

Mechanical Advantage– p.416-417

A machine’s mechanical

advantage is the number of times

a machine increases force exerted on it.

Efficiency of Machines – p.417-419

A machine’s efficiency (expressed as a percent)

compares the output work to the input

work.Some work is always wasted overcoming the force of friction.

Efficiency = Output work x 100% Input work

Simple Machines p.422

Inclined Plane p.423

• A flat, sloped surface• Ex: ramp• Allows you to exert your

input force over a longer distance.

• Ideal mechanical advantage =

Length of incline Height of incline

Wedge p.424

• A device that is thick at one end and tapers to a thin edge at the other end.

• 2 inclined planes back to back that can move.

• Ex: ax, knife, zipper• Allows the output force at a

90o angle to the slope• Ideal mechanical advantage = Length of wedge Width of wedge

Screw p.425

• An inclined plane wrapped around a cylinder.

• Ex: lids of jars• Threads of screw increase the

distance over which you exert the input force. As threads of screw turn, they exert an output force, holding the object in place.

• Ideal mechanical advantage = Length around the threads Length of screw

Levers p.426-427

• A rigid bar that is free to pivot, or rotate on a fixed point, called the fulcrum.

• Ex: wheelbarrow, hockey stick, crowbar

• 3 types (see next slide)• Ideal mechanical

advantage = Distance from fulcrum to input force Distance from fulcrum to output force

1st class levers p. 427

• Change the direction of the input force.

• If the fulcrum is closer to the output force, these levers also increase force.

• If the fulcrum is closer to the input force, these levers also increase distance.

• Examples: scissors, pliers, seesaws

2nd class levers p. 427

• Change the direction of the input force.

• If the fulcrum is closer to the output force, these levers also increase force.

• If the fulcrum is closer to the input force, these levers also increase distance.

• Examples: scissors, pliers, seesaws, wheelbarrow

3rd class levers p. 427

• Increase distance, but do not change the direction of the input force.

• Examples: fishing poles, shovels, baseball bats, hockey sticks, brooms

Wheel and Axle p.428-430• Two circular or cylindrical objects

fastened together that rotate about a common axis.

• Ex: screwdriver, doorknob, steering wheel

• You apply an input force to turn the handle, or wheel. Because the wheel is larger than the axle, the axle rotates and exerts a large output force. The W&A increases your force, but you must exert your force over a long distance.

• Ideal mechanical advantage = Radius of wheel Radius of axle

Pulley p.430-431

• A grooved wheel with a rope or cable wrapped around it.

• Ex: flagpole, crane, weight machine, blinds/drapes

• You pull on one end of a rope (input force), while the output force pulls the object you want to move. Makes work easier in 2 ways (see next slide)

• Ideal mechanical advantage = number of sections of rope that

support the object.

Simple Machines in the Body p.432

• Most of the machines in your body are levers that consist of bones and muscles

•Your front teeth (incisors) are wedges!

Which levers are found in your body?

Compound Machines p.433

• Utilize two or more simple machines

• Ideal mechanical advantage is the product of the individual ideal mechanical advantages of the simple machines that make it up.