Work and EnergyWork and Energy

Chapter 5Chapter 5

WorkWork

Definition of WorkDefinition of Work

What do you think?What do you think?

Work Work is done on an object when a force is done on an object when a force causes a displacement of the object.causes a displacement of the object.

Work changes the amount of energy in Work changes the amount of energy in an object!an object!

Pushing Your CarPushing Your Car

Imagine your car has ran out of gas.Imagine your car has ran out of gas. You have to push.You have to push. If you push with a constant horizontal If you push with a constant horizontal

force then the work done is equal to the force then the work done is equal to the magnitude of the force times the magnitude of the force times the magnitude of displacement of the car.magnitude of displacement of the car.

W = F*W = F*ΔΔxx

Is Work Done When . . .Is Work Done When . . .

If you hold a chair at arms length is work If you hold a chair at arms length is work done on the chair?done on the chair?

NOOOOOOOOOOOOOOOOOOO!NOOOOOOOOOOOOOOOOOOO! Work is not done on an object unless the Work is not done on an object unless the

object is moved with the action of a force.object is moved with the action of a force. The muscles in your tired arms are doing The muscles in your tired arms are doing

work inside your body, but not on the work inside your body, but not on the chair.chair.

WorkWork

WorkWork is done only when components of a force is done only when components of a force are parallel to a displacement.are parallel to a displacement.

Imagine pushing a crate along the ground.Imagine pushing a crate along the ground. If all your force is horizontal, all your effort If all your force is horizontal, all your effort

moves the crate.moves the crate. If your force is at an angle, only the horizontal If your force is at an angle, only the horizontal

component of your force moves the crate and component of your force moves the crate and contributes to work.contributes to work.

Push the table!Push the table!

Push the table you are sitting at (or chair Push the table you are sitting at (or chair depending)depending)

11stst push it by applying a force at a push it by applying a force at a downward angle.downward angle.

Next push it back by applying a force that Next push it back by applying a force that is completely horizontal.is completely horizontal.

How did the forces compare?How did the forces compare?

Example:Example:

While climbing a horse to go horseback While climbing a horse to go horseback riding, you are applying a force that will riding, you are applying a force that will pull you up on top of the horse. You are pull you up on top of the horse. You are doing work to climb up because the doing work to climb up because the direction of the force is parallel to the direction of the force is parallel to the direction of the motion.direction of the motion.

WorkWork

Your turn! Take two minutes to come up Your turn! Take two minutes to come up with an example of work that you have with an example of work that you have experienced at some point this week.experienced at some point this week.

Write it down in your lab book, then share Write it down in your lab book, then share your idea with a partner.your idea with a partner.

Check to see if each others example is in Check to see if each others example is in fact an example of work.fact an example of work.

CrateCrate

WorkWork

If If θθ = 0° then cos 0° = 1 and = 0° then cos 0° = 1 and W = Fd.W = Fd. If If θθ = 90° then cos 90° = 0 and = 90° then cos 90° = 0 and W = 0.W = 0. So no work is done on the crate when So no work is done on the crate when

force is applied downward at a 90° angle.force is applied downward at a 90° angle. Work = F*Work = F*ΔΔxx θθ If many forces are acting on an object we If many forces are acting on an object we

can find the net work done.can find the net work done. WorkWorknetnet = F = Fnetnet**ΔΔx*cosx*cos θθ

Work DimensionsWork Dimensions

Work has the dimensions of force x Work has the dimensions of force x length.length.

Which is N x m (Newton’s x meters)Which is N x m (Newton’s x meters) Also known as Also known as Joules (J)Joules (J) What is a Joule?What is a Joule? 1 J is about the amount of work it take to 1 J is about the amount of work it take to

life an apple from your waist to the top of life an apple from your waist to the top of your head.your head.

ExampleExample

Gen:Gen: How much work is done on a vacuum How much work is done on a vacuum

cleaner pulled 3.0 m by a force of cleaner pulled 3.0 m by a force of 50.0 N?50.0 N?

Adv:Adv: at an angle of 30.0° above the at an angle of 30.0° above the

horizontal?horizontal?

SolutionSolution

Gen: Gen: Work=F*Work=F*ΔΔxx Plug and chugPlug and chug W = (50 N)*(3.0 m)W = (50 N)*(3.0 m) W= 150 JW= 150 J Adv: Adv: Work=F*Work=F*ΔΔx *cosx *cos θθ Plug and chugPlug and chug W = (50.0 N)(3.0 m)(cos 30.0°)W = (50.0 N)(3.0 m)(cos 30.0°) W = 130 JW = 130 J

Your TurnYour Turn

Gen: Gen: A man pulls a horse with a constant net A man pulls a horse with a constant net horizontal force of 172.0 N and causes the horizontal force of 172.0 N and causes the horse to move through the stables. How much horse to move through the stables. How much work is done if the horse moves a distance of work is done if the horse moves a distance of 13.00 m?13.00 m?

Adv:Adv: In reality, the man is pulling the horse at In reality, the man is pulling the horse at a downward angle of 25° with a constant force a downward angle of 25° with a constant force of 172.0 N, which cause the horse to move of 172.0 N, which cause the horse to move through the stables. How much work is done if through the stables. How much work is done if the horse moves a distance of 13.0 m?the horse moves a distance of 13.0 m?

The Sign of WorkThe Sign of Work

The sign of work is important.The sign of work is important. Positive or negative.Positive or negative. Work is positive when the force is in the Work is positive when the force is in the

same direction as the displacement.same direction as the displacement. Work is negative when the force is in the Work is negative when the force is in the

direction opposite of the displacement.direction opposite of the displacement.

Example:Example:

When throwing a lacrosse ball, the ball When throwing a lacrosse ball, the ball would be experiencing a positive amount would be experiencing a positive amount of work.of work.

When catching a lacrosse ball, the ball When catching a lacrosse ball, the ball would be experiencing a negative would be experiencing a negative amount of work.amount of work.

Your turn:Your turn:

Write in your lab book an example of Write in your lab book an example of positive/negative work?positive/negative work?

Share your idea with your neighbor.Share your idea with your neighbor.

Graphing!Graphing!

Work can also be solved utilizing Work can also be solved utilizing graphing methods. graphing methods.

In this image, where is In this image, where is the work?the work?

W = F * W = F * ΔΔx so…….x so…….

Work is the are under the curve in a Work is the are under the curve in a Force vs. Displacement graph.Force vs. Displacement graph.

What are the different What are the different types of energy in the types of energy in the universe?universe?

Take a minute!Take a minute!

Do a quick google search and try to Do a quick google search and try to identify all of the different types of energy identify all of the different types of energy out there!out there!

Create a list with a brief description in Create a list with a brief description in your lab book.your lab book.