Work and Energy Chapter 5

Work

Definition of Work  What do you think?  Work is done on an object when a force causes a displacement of the object.  Work changes the amount of energy in an object!

Pushing Your Car  Imagine your car has ran out of gas.  You have to push.  If you push with a constant horizontal force then the work done is equal to the magnitude of the force times the magnitude of displacement of the car.  W = F*Δx

Is Work Done When...  If you hold a chair at arms length is work done on the chair?  NOOOOOOOOOOOOOOOOOOO!  Work is not done on an object unless the object is moved with the action of a force.  The muscles in your tired arms are doing work inside your body, but not on the chair.

Work WWWWork is done only when components of a force are parallel to a displacement. IIIImagine pushing a crate along the ground. IIIIf all your force is horizontal, all your effort moves the crate. IIIIf your force is at an angle, only the horizontal component of your force moves the crate and contributes to work.

Push the table!  Push the table you are sitting at (or chair depending)  1 st push it by applying a force at a downward angle.  Next push it back by applying a force that is completely horizontal.  How did the forces compare?

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

Work YYYYour turn! Take two minutes to come up with an example of work that you have experienced at some point this week. WWWWrite it down in your lab book, then share your idea with a partner. CCCCheck to see if each others example is in fact an example of work.

Crate

Work  If θ = 0° then cos 0° = 1 and W = Fd.  If θ = 90° then cos 90° = 0 and W = 0.  So no work is done on the crate when force is applied downward at a 90° angle.  Work = F*Δx θ  If many forces are acting on an object we can find the net work done.  Work net = F net *Δx*cos θ

Work Dimensions WWWWork has the dimensions of force x length. WWWWhich is N x m (Newton’s x meters) AAAAlso known as Joules (J) WWWWhat is a Joule? 1111 J is about the amount of work it take to life an apple from your waist to the top of your head.

Example  Gen:  How much work is done on a vacuum cleaner pulled 3.0 m by a force of 50.0 N?  Adv:  at an angle of 30.0° above the horizontal?

Solution  Gen: Work=F*Δx  Plug and chug  W = (50 N)*(3.0 m)  W= 150 J  Adv: Work=F*Δx *cos θ  Plug and chug  W = (50.0 N)(3.0 m)(cos 30.0°)  W = 130 J

Your Turn  Gen: A man pulls a horse with a constant net horizontal force of N and causes the horse to move through the stables. How much work is done if the horse moves a distance of m?  Adv: In reality, the man is pulling the horse at a downward angle of 25° with a constant force of N, which cause the horse to move through the stables. How much work is done if the horse moves a distance of 13.0 m?

The Sign of Work  The sign of work is important.  Positive or negative.  Work is positive when the force is in the same direction as the displacement.  Work is negative when the force is in the direction opposite of the displacement.

Example:  When throwing a lacrosse ball, the ball would be experiencing a positive amount of work.  When catching a lacrosse ball, the ball would be experiencing a negative amount of work.

Your turn:  Write in your lab book an example of positive/negative work?  Share your idea with your neighbor.

Graphing!  Work can also be solved utilizing graphing methods.

In this image, where is the work?

W = F * Δx so…….  Work is the are under the curve in a Force vs. Displacement graph.

What are the different types of energy in the universe?

Take a minute!  Do a quick google search and try to identify all of the different types of energy out there!  Create a list with a brief description in your lab book.