Chapter 8 Guided Reading

_____________ occurs when a force causes an object to move in the direction of the force. Work is done on an object only when a force makes that object _______________. WORK MOVE

Work is done on an object if two things occur: (1) the object moves as a _________________ is applied and (2) the direction of the object's motion is the ________________ as the direction of the force applied. SAME FORCE

Work = ________ X _________ unit for Force ____________ unit for distance ____________ FORCE DISTANCE Newton Meter The amount of work (W) done on moving an object can be calculated with the following formula:

the unit used to express work is the newton-meter (Nm), which is more simply called the _______________ (J). Joule

A man applies a force of 500 N to push a truck 100 m down the street. How much work does he do?

Work = ________ X _______ = ______________________ (don’t forget the unit of measurement) 50,000 joules 100 M500 N

In which situation do you do more work? (circle the correct answer) a. You lift a 75 N bowling ball 2 meters off the floor. b. You lift two 50 N bowling balls 1 meter off the floor.

Power is the rate at which ________________ is done. WORK

Formula to calculate power: Power = _____________ Units: Time: ____________ Work: ______________ ) WORK SECONDS JOULES TIME

The unit used to express power is joules per second (J/s), which is more simply called the _______________ WATT

A machine is a device that helps make work easier by changing the _________or _______________of a force. SIZEDIRECTION

Two kinds of work are always involved when a machine is used- the work done on the _________________and the work the machine does on___________ ______________. MACHINE ANOTHER OBJECT

You apply a force, called the _____________ __________________, to the machine and move it through a distance. INPUT FORCE

The machine applies a force, called the_________________ _________________, through a distance. OUTPUT FORCE

Machines help you because they increase the amount of work done. (True / False)

Machines help you because they DECREASE the amount of work done. (True / False) EXTRA QUESTION

Work output can never be greater than work input. (True / False)

Machines make work easier because they change the ___________ or _______________ of the input force. SIZE DIRECTION

A ramp decreases the amount of ______________ necessary to do the work of lifting the box. FORCE HUSH

But the ______________________ over which the force is exerted increases. In other words, the machine allows a smaller ____________ to be applied over a longer distance. DISTANCE FORCE

The Force-Distance Trade-off

When a machine changes the size of the force, the distance through which the force is exerted must also change.

Force or distance can increase, but not together. When one increases, the other must _____________________. DECREASE

This is because the work output is never greater than the work ______________. INPUT

Do some machines make work easier than others?

Yes, because some machines can increase ____________ more than others. FORCE

A machine's mechanical advantage tells you how many times the machine multiplies _______________. FORCE

Formula to calculate Mechanical advantage (MA) = _________________ OUTPUT FORCE INPUT FORCE

Take a look at Figure 8 on page 196. In this example, the output force is greater than the input force. Using the equation above, you can find the mechanical advantage of the handcart:

M A = _____________ = 500 N 50 N 10

Because the mechanical advantage of the handcart is __, the output force is ___ times bigger than the input force. 10 STOP GASPING

The larger the mechanical advantage, the __________ a machine makes your work. But as mechanical advantage increases, the distance that the output force moves the object ___________________. EASIER DECREASES

Remember that some machines only change the direction of the force. In such cases, the output force is equal to the input force, and the mechanical advantage is____. 1

The less work a machine has to do to overcome friction, the more ________________________ it is. EFFICIENT

A machine's mechanical efficiency is calculated using the following equation:

M E = ____________ X WORK OUTPUT WORK INPUT 100 BE GOOD

No machine is 100 percent efficient, but reducing the amount of _______________ in a machine is a way to increase its mechanical efficiency. FRICTION

Section 3 Types of Machines

A lever is a simple machine consisting of a bar that pivots at a fixed point, called a ____________________. FULCRUM

There are three classes of levers, based on the locations of the fulcrum, the load, and the input force.

With a first class lever, the fulcrum is between the _____________ ___________and the __________, INPUT FORCE LOAD

First class levers always change the ____________________ of the input force. DIRECTION

And depending on the location of the fulcrum, first class levers can be used to increase __________or to increase ________________. FORCE DISTANCE PAY ATTENTION

When the fulcrum is closer to the load than to the input force, a mechanical advantage of _______________ than 1 results. The output force is ___________________ because it is exerted over a shorter distance. GREATER

When the fulcrum is exactly in the middle, a mechanical advantage of _____________ results. The output force is not increased because the input force's distance is not increased. 1

When the fulcrum is closer to the input force than to the load, a mechanical advantage of ______ _______1 results. Although the output force is less than the input force, a gain in distance occurs. LESS THAN

With a second class lever, the load is between the ______________ and the _____________ ___________. FULCRUM INPUT FORCE

Second class levers do not change the direction of the input force, but they allow you to apply less force than the force exerted by the load.

Because the output force is greater than the input force, you must exert the input force over a greater distance.

Using a second class lever results in a mechanical advantage of greater than 1.

The closer the load is to the fulcrum, the more the _______________ is increased and the greater the mechanical advantage. FORCE

With a third class lever, the input force is between the fulcrum and the ________________. LOAD

Third class levers do not change the _____________________ of the input force. DIRECTION

In addition, they do not increase the input force. Therefore, the output force is always less than the input force.

An inclined plane is a simple machine that is a straight, slanted ___________________. SURFACE

A __________________ is an example of an inclined plane. RAMP

The ___________________ the inclined plane is compared with its height, the greater the mechanical advantage. LONGER

A wedge is a double inclined plane that ___________________. MOVES

A wedge applies an output force that is greater than your input force, but you apply the input force over a greater_____________________. DISTANCE

The longer and thinner the wedge is, the greater the _____________________ _____________________. MECHANICAL ADVANTAGE

A screw is an inclined plane that is wrapped in a ________________. SHAFT

When a screw is rotated, a _______________ force is applied over the ________________ distance along the inclined plane of the screw. SMALL LONG

A doorknob is an example of a ____________ and ___________, a simple machine consisting of two circular objects of different sizes. WHEELAXLE

Because the axle is smaller than the wheel, it rotates through a _________________ distance, which makes the output force ________________ than the input force. SHORTER GREATER

A pulley is a simple machine consisting of a grooved ______________that holds a rope or a cable. WHEEL

There are two kinds of pulleys- fixed and movable.

Fixed and movable pulleys can be combined to form a ________________ and __________________. BLOCK TACKLE

Some pulleys only change the direction of a force. This kind of pulley is called a ________ pulley. FIXED

Movable pulleys do increase force, but you must exert the input force over a greater _____________________ than the load is moved. DISTANCE HUSH

Compound machines, machines that are made of two or more __________________ machines. SIMPLE BE QUIET

The more moving parts a machine has, the lower its ______________________ _____________________. MECHANICAL EFFICIENCY

For compound machines that involve many simple machines, such as automobiles and airplanes, it is very important that _______________________ be reduced as much as possible through the use of lubrication and other techniques. FRICTION