Unit 4: Energy, heat and motion

Work and Power

What is work?

• Definition: work occurs when a force causes an object to move in the direction of the force.

• characteristic – the direction the object moves MUST be in the same direction of FORCE to be considered WORK

• Ex. • Force

• Distance it moves

Work or not work? Example Direction of force Direction of

movementDoing work? (yes/no)

Josh pushing a box across the floor.Symone carrying her books to her next classMs. Tran lifting a box off the groundDe’Andre placing a box on the ground

How do we calculate work?

• Formula: work = force x distance• Ex. Find the work done on the box if a force of

55N was applied to the right and the box moved a distance of 10m.

• 55N• 10M

• Is work being done on this object?

Work out the problem

• Work = force x distance• Work = 55N x 10m• Work = 550 Newton-meters or JOULES (J)

Practice Problem #1

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

Practice Problem #2

• In which situation do you do more work? (Calculate the work done in each scenario)– A. you lift a 75N bowling ball 2m off the floor. – B. You lift two 50N bowling balls 2m off the floor.

Energy

• DEFINITION: Energy is the ability to do work.• Energy is all around you! You can hear energy

as sound. You can see energy as light. You can feel energy as wind. You use energy to bounce a basketball or lift a backpack.

Energy & Work

• What do energy and work have to do with each other?

• Energy and Work are directly related…in fact, they are both measured in JOULES!

• Objects can gain energy because work is being done on them. Can you think of any examples?

Energy & Work: Examples

• When you kick a football, you give some of your energy to the football to make it move.

• When you throw a bowling ball, you give it energy to make it roll.

• When the bowling ball hits the pits, it loses some of its energy to the pins, causing them to fall!

Power

• What is power?

Power

• DEFINITION: Power is the rate at which work is done.

• FORMULA: Power = Work ÷ Time• Or: Power = Energy ÷ Time• Or: (Force x Distance) ÷ Time• Unit of measure: Watt (W)

Power: Let’s Practice…

• An Antarctic explorer uses 6000 Joules of work to pull his sled for 60 seconds. What power does he need?

• Power = Work ÷ Time• Work = 6000J, Time = 60sec• Power = 6000J ÷ 60sec = 100 Watts

Power: Let’s Practice…

• If a conveyor belt at a candy factory uses 10 J to move a piece of candy a distance of 3 meters in 20 seconds, what is the conveyor belt’s power?

• Power = Work ÷ Time• Work = 10 J, Time = 20 sec• Power = 10 J ÷ 20 sec = 0.5 Watts

Light Bulbs

• But wait! Aren’t watts used to describe light bulbs???

• Yes! For example, a 60-watt light bulb uses 60 joules of energy every second to shine!

Power: Let’s Practice…

• A light bulb uses 600 J of energy in 6 seconds. What is the power of the light bulb?

• Power = Energy ÷ Time• Energy = 600 J, Time = 6 sec• Power = 600 J ÷ 6 sec = 100 watts

More Practice…

• What two factors do you need to know in order to calculate how much work was done in any situation?

• Answer: You need to know FORCE and DISTANCE.

More Practice…

• If you push very hard on an object but it does not move, have you done work? Why or why not?

• Answer: NO! The object must move for work to be done.

More Practice…

• Was work done on a book that fell from a desk to the floor? If so, which force was involved?

• Answer: YES! The force of GRAVITY was involved.

More Practice…

• Work is done on a ball when a soccer player kicks it. Is the player still doing work on the ball as it rolls across the ground? Why or why not?

• NO! The player is no longer doing work on the ball while it is rolling. The ball keeps rolling because of inertia.

More Practice…

• How is Power related to Work?

• Answer: Power is the rate at which work is done. The faster you do work, the greater your power.

More Practice…

• By increasing the SPEED at which you work, you can increase your:

a) Force b) Workc) Energy d) Power

Answer: d) Power

More Practice…

• Which takes more power: using 15 Newtons to lift a ball 2 meters for 5 seconds, or using 100 Newtons to push a box 2 meters in 50 seconds?

• Answer: The first situation. • Power = (15N x 2m) ÷ 5 sec = 6 W• Power = (100N x 2m) ÷ 50 sec = 4W– 6 Watts is greater than 4 Watts

More Practice…

• How can you tell if a force you apply to an object is doing work?

• Answer: You can tell the force is doing work because the object will MOVE.

More Practice…

• How much work is done if you drag a box weighing 150 Newtons a distance of 30 meters?

• Work = Force x Distance• Force needed to drag suitcase = 150N• Distance = 30m• Work = 150N x 30m = 4,500 Joules (J)

More Practice…

• A light bulb uses 400 J of energy in 8 seconds. What is the power of the light bulb?

• Power = Energy ÷ Time• Energy = 400 J, Time = 8 sec• Power = 400 J ÷ 8 sec = 50 watts

Exit Ticket: So What Did We Learn?

• What is WORK?• What is ENERGY?• What is POWER?• How do they all relate to what we’ve already

learned about FORCE?