What is Work?

Work Demo

• One student pushes against the wall.

• One student pulls up a set of keys

Who does more work? Why?

Who was applying more force?

What is force?

What is work?

Force

A force is a push or pull that can cause a mass to move.

Force is measured in newtons, (N).

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When you kick (force) a ball (mass) it moves.

What is work?

• Ms. Hartman thinks…– Work is the amount of force required over a

certain amount of distance.

• What do you think work is?– Write at LEAST 2 sentences in your journals

about what you think work is.

Work

Work is done when a force acts on an object to move it a certain distance in the same direction.If you push a mass for a certain distance, you have done a certain amount of work.

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Work is measured in joules, (J).

Work Facts

1) Measured in Joules (J)

2) If the object does NOT move, there is NO WORK

3) The object must move in the SAME direction as the force

What takes more work?

A.) Mowing the front yard of a house

B.) Mowing the grass of a baseball stadium

Which one will take more work?

A.) Pushing a bus 100 meters.

B.) Pushing a car 100 meters.

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Work is done by the mouse

Work or Not Work?

Is the woman doing work on the box by pushing it across the floor?

Work or Not Work?

Is the woman doing work on the backpack by carrying it across the room?

Work or Not Work?

Is the woman doing work by lifting the bag off of the floor?

Work or Not Work?

Is the woman doing work on the bag of groceries by carrying the bag across the room?

No Work is done!Regardless of the amount of effort or force used, if the rock doesn't move,the person does no work.

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Work or Not Work?• A scientist delivers a speech to an

audience of his peers. • A body builder lifts 350 pounds

above his head. • A mother carries her baby from room

to room. • A father pushes a baby in a carriage.

• A woman carries a 20 kg grocery bag to her car?

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• A scientist delivers a speech to an audience of his peers. NoNo

• A body builder lifts 350 pounds above his head. YesYes

• A mother carries her baby from room to room. NoNo

• A father pushes a baby in a carriage. YesYes

• A woman carries a 20 km grocery bag to her car? NoNo

Work or Not Work?

Is there any work? Answer the following situations.

An object is moving at a constant speed of 20km/h. You did not apply any force to the object, the object was already moving. Have you done any work to the object?

You are pushing a wall with a force of 200N. The wall does not move.Have you done any work to the object?

Work

The relationship between work and force is simply that work = force × distance moved

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If you push the mass twice as far, you will have done twice as much work.

If you push a mass with a force of 300 N over a distance of 10 m.300 N × 10 m = 3,000 JYou have done 3,000 J of work.

List the situations from most work to least amount of work.

1. You are pushing a box with a force of 20N a distance of 100 meters.

2. You are pushing a box with a force of 20N a distance of 0 meters.

3. You are pushing a box with a force of 20N a distance of 110 meters.

4. You are pushing a box with a force of 20N a distance of 90 meters.

Answers

• 3-1-4-2

• Each problem showed the same amount of force being applied, but they had a different amount of distance being covered. This created a different amount of work.

Practice Problem 1• Victor pushes a box with a force of 15N over a

distance of 30m. What is the amount of work done?

Known Formula Substitution Answer

Practice Problem 2• If you push a mass with a force of 300 N

for 20m, how much work have you done?

Known Formula Substitution Answer

Which of the following increases the amount of work done?

Question 3

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A) Increasing the distance moved

B) Increasing the force used to push it

C) Pushing the object up a steep hill

D) All of the above

Solving For Different Variables

Question: What is the formula to calculate for work?

Work

• If I am looking for work and I know distance and force, what equation am I going to use?

W= F x D

Force

• If I am looking for force and I know distance and work, what equation am I going to use?

F = W / D

REMEMBER: TIBO

Distance

• What equation will I use if given work and force but not distance?

D = W / F

REMEMBER: TIBO

Practice Problem 3• Gerardo drags his dog around the block with a

force of 50N. This takes 100J of work for him to complete this task. What is the distance around the block?

Known Formula Substitution Answer

Practice Problem 4• Miriam drives a car for 400m. It takes Miriam

1200J to do this. How much force did Miriam and her car exert?– Copy this problem, then list what we know and don’t

know.• F=• W=• D=

– Then tell me what equation we need to use to solve this problem.

But can’t we make work easier?

YES! By using simple machines!

What is purpose of a machine?• To make work easier.

*** Simple machines do NOT do less work. They allow a person to use less force to do the same amount of work. If less force is used, and work is constant, then distance must increase. Each simple machine increases the distance over which the effort force is applied.***

How do machines make work easier?

• They can change the amount of force

• They can change the distance

(Remember that Work = Force X Distance)

• They can also change the direction

MORE

How do machines make work easier?

Any change in the size of the force changes the distance. No machine can increase both force and distance.

1. Multiply force which decreases distance the load moves.

2. Multiply the distance which decreases force.3. Or, they leave force and distance alone but change

the direction in which the load moves.

What are the 6 types of simple machines?

• There are three simple machines:

• The lever, the pulley, and the inclined plane The wheel and axle, the wedge, and the screw are modifications of these simple machines.

1. Inclined plane

A straight slanted surface.

( Example = a ramp.)

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Although it takes less force for car A to get to the top of the ramp, all the cars do the same amount of work.

A B C

2. LEVER

A rigid bar free to pivot (move) around a fixed point called a fulcrum

3. PULLEYS

A rope, belt or chain wrapped around a grooved wheel and change the direction of a force or it can change the amount of force.

4. Wheel and Axle

A modification of a pulley. A wheel is fixed to a axle and they move together

5. Wedge

Two inclined planes put together.

6. Screw

An inclined plane that wraps around a shaft.

Question 1

Question 2

7 Newtons

Question 3

2.5 Newtons

Question 4

7.5 meters

2 meters

Question 5