(Are they really that simple?!)

transcript

What have we learned by doing labs and researching for our notes?

What is work?• In science, the word work has a

different meaning than you may be familiar with.

• #1 According to your text, work is done when a force causes an object to move in the direction of the force

Work or Not?• According to the

scientific definition, what is work and what is not?– a teacher lecturing

to her class– a mouse pushing a

piece of cheese with its nose across the floor

Section 1 What is Work?Chapter 8

2. What is a “force”?

• A push or a pull

What unit describes the size of a force?

• Newton (N) Measured with a spring scale

3. What is a machine?

• Something that makes work easier

4. How do machines make work easier? What do machines do?

• They change the size or direction of the force you apply

5. The work YOU do on a machine is called the• Work input (input force) OR effort

The work done by the machine on another object is called the

• Work output (output force)

work input or effort

work output

6. Machines DO NOT change the amount of

work done.

They only make it

• Work = F (force) X d (distance)

easier.

12 J = 3N x 4m12 J = 2N x 6m12J = 1N x 12m

So, why use machines?

Machines make work easier by changing the• amount of force you exert• distance over which you exert your force, or• direction in which you exert your forceHowever, the amount of workdoes not change! (W=Fxd)

TIME OUT!!! What is “mechanical advantage”?

• Jot this down at the bottom of your page!

Mechanical advantage is the number of times a machine multiplies force.

• The greater the MA, the less effort required and the easier the work!

• And that’s a GOOD thing!

Inclined plane p. 145

Describe:

• Flat, slanted surface.

An inclined plane makes work easier by lettingyou apply a _____________ force over a __________________

smallerlonger distance

The longer the ramp, theless effort required, thegreater the mechanicaladvantage.

Inclined Plane: Examples

• ramp• stairs• hills• boat ramps• winding

roads • slides

Which inclined plane has the greater mechanical advantage? Why?

Which machine above makes less work?

Did I getcha? Work is the same; it just gets easier!

Why? Shorter distance, more effort. 2m x 6N = 12J Longer distance, less effort. 6m x 2N = 12 J

See your lab: Exploring Inclined Planes!

Wedge p. 146

Describe: • Two inclined planes that move

Like the inclined plane, the wedge needs a __________ input force over a _________________

smalllarge distance

Which wedge has the greater mechanical advantage?

The longer and thinner, the greater the mechanical advantage.

That’s why sharper knives work better for cutting!

Wedge: Examples

• nails• knives• chisels• axes• paper cutter• can opener• needle/pin• door stop

Screw p. 146

Describe:

• An inclined plane wrapped around a cylinder

To turn a screw, a ____________ force over a ______________is needed.

smallergreater distance

The closer the threads, the greater the mechanical advantage.

Which screw has the greatest mechanical advantage?

• Remember to look at the pitch-how close the threads are!

How about these?

Screw: Examples

• screws• light bulbs• bolts• jar lids• C-clamps• corkscrews• drill bits

1st class lever p. 141

Describe a lever:

What is a fulcrum? A fixed point

FLE: fulcrum

The closer the fulcrum to the load, the greater the mechanical advantage.

What’s in the middle?

Resistance is AKA the load

A bar that rotates at a fixed point

*Change in direction

1st lever: Examples

• see saw• paint can opener• crow bar• door on hinges

See Lab: Exploring Levers

2nd class lever p. 142

The output force does work on the load

FLE: load (or resistance)

*No change in direction

2nd class lever: Examples

• wheelbarrow• nutcracker• garlic press

See Lab sheet

3rd class lever p. 142

The input force is AKA as your effort

FLE: effort

*No change in direction

3rd class lever: Examples

• hammer• broom• fishing pole

See Lab sheet

Wheel and axle p. 144

Describe:

• Two rounded objects that move together

The larger object is the _____________ The smaller object is the ______________

The larger the wheel, the longerthe distance and the greater the MA.

Some wheels are NOT simple machines. They only reduce friction between two surfaces that are rubbing together. Let’s check your worksheet now to see if you understand the difference.

reduce friction wheel and axle

Refer to your worksheet on wheels.

Wheel and axle p. 144

Gears are a special type of wheel and axle. They have teeth that interlock with other gears.

Wheel and axle: Examples

• faucet• doorknobs• steering wheels• screwdrivers• wrenches• egg beaters

pulley p. 143

Describe:

• a grooved wheel that holds a rope or cable

What are pulleys used for? lifting

Fixed pulley p. 143

To determine the MA of a pulley,count the support ropes thatare pulling UP.

MA = 1This pulley only changesThe direction of the force.See Lab: Exploring Pulleys

Movable pulley p. 143

Count the support ropes pullingUP to determine the MA.

MA = 2This pulley multiplies yourforce by two. You need half the effort. 20 N

10 Nforce

Block and tackle pulley system p. 143

Count the support ropesto determine the MA.

MA = 4This pulley multiplies yourforce by four. You need one-fourth the effort.

20 Nblock

5 Nforce

What is the MA of the compound pulley we used in lab?

MA = 3This pulley multiplies yourforce by three. You need one-third the effort.

Pulley: Examples

• mini-blinds• hoists in a garage• flag poles• sailing ships

A fixed pulley is shown. A fixed pulley can make work easier by

A. Changing the size of the input force needed to lift an object

B. Changing the direction of the force needed to lift an object

C. Changing the size and direction of the force needed to lift an object

D. Changing the amount of work that must be done

A fixed pulley is shown. A fixed pulley can make work easier by

A. Changing the size of the input force needed to lift an object

B. Changing the direction of the force needed to lift an object You pull down, object goes UP. MA = 1

C. Changing the size and direction of the force needed to lift an object

D. Changing the amount of work that must be done NEVER!

This moveable pulley makes work easier by

A Changing the size of the input force needed to lift an object

B Changing the direction of the force needed to lift an object

C Changing the size and direction of the force needed to lift an object

D Changing the amount of work that must be done

This moveable pulley makes work easier by

A Changing the size of the input force needed to lift an object You pull UP, the block moves UP. However, there are 2 ropes pulling UP, so the MA = 2. (change in force)

D Changing the amount of work that must be done NEVER!!

The man in the picture is using a pulley system or a block and tackle. A pulley system can make work easier by

D Changing the amount of work that must be done

The man in the picture is using a pulley system or a block and tackle. A pulley system can make work easier by

C Changing the size and direction of the force needed to lift an object Pull down, object UP. Two support ropes, MA = 2, half the effort force is needed to lift the block.

D Changing the amount of work that must be done NEVER!

(Are they really that simple?!)Sure!! Now that you have learned all about them!

Study your notes tonight!

(Are they really that simple?!)

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