OBJECTIVE/LEARNING GOAL(S) 1. I will know the difference between the non-scientific and scientific definition of work. 2. I will know the difference between work and power. 3. I will be able to mathematically solve story problems involving work and power.
☻ We can easily say that the nonscientific understanding for work is like a job, or sitting and working on your research paper.
☻ In science, work is the product of force and distance. ☻ What does the word product mean? ☻ Multiplication
☻ Is there a force? YES (There is a kick.) ☻ Is there distance? YES (The crate starts and stops at different points.) ☻ Is work done? (Work = Force x distance) YES
The weight lifter applies a large force to hold the barbell over his head. Is the barbell moving? NO IS THERE WORK BEING DONE WHEN HE HOLDS IT UP IN THE AIR? NO NO DISTANCE – NO WORK IS BEING DONE!
IS WORK BEING DONE ? E XPLAIN HOW YOU KNOW. A B
P ARTNER PRACTICE – ANSWER WORK IS BEING DONE! FORCE and distance are in the SAME direction! Force distance
work= force x distance Equation: W = F x d
W = F x d What other equation uses W? W = mg What does W mean? Weight!
What is the unit for force? newton What is the unit for distance? meter What is the unit for work then? newton ∙ meter N∙m
The joule (J) is the SI unit of work. A J is equal to 1 N*m JOULE! Science gives NEWTON-METER A SPECIAL NAME
A weight lifter raises a 1600 N barbell to a height of 2.0 meters. How much work was done? Givens: 1600 N = F 2.0 meters = d Unknown: Work Equation: W = Fd Plug in numbers: W =1600 N × 2.0 m Work = 3,200 N∙m A B
A karate expert kicks a crate with a force of 2,250 N. The crate moves a displacement of 5 m. How much work was done? W = 11,250 N∙m Givens: 2,250 N = F 5.0 m = d Unknown: Work Equation: W = Fd Plug in numbers: W =2,2500 N × 5.0 m
How are work and power related? Power is the rate of doing work. Doing work at a faster rate requires more power.
Work is required to move snow from one location to another. One person uses a shovel and another uses a snow blower. Does one have more or less power than the other? Explain. A B
Because the snow blower can remove more snow in less time, it requires more power than hand shoveling does.
Power = Work time P = W t
What is the unit for Work? Joule What is the unit for time? second What is the unit for power then? Joule s
The SI unit of power is the watt (W), which is equal to one joule per second. WATT! Science gives Joule/s A SPECIAL NAME
You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box?
Givens: 72 newtons = F 1.0 meters = d 2 seconds = t Unknown: Power Equation: P = W/t P = Fd/t Plug in numbers: P = (72 N)(1 m) ÷ 2 s Power = 36 J/s P = 36 W
Is this answer reasonable? 36 watts is not a lot of power, which seems reasonable considering the box was lifted slowly, through a height of only 1 meter.
Your family is moving to a new apartment. While lifting a box 1.5 m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this?
Givens: 200 newtons = F 1.5 meters = d 2 seconds = t Unknown: Power Equation: P = W/t P = Fd/t Plug in numbers: P = (200 N)(1.5 m) ÷ 1 s Power = 300 J/s P = 300 W
FUN FACT: The Watt is named after James Watt ( ).
Work is the product of force in the direction of displacement. SI Unit of work is joule or newton.meter. Power is the rate of work. SI Unit of power is watt or joule per second.
1. A tractor exerts a force of 20,000 newtons to move a trailer 8 meters. How much work was done on the trailer? a. 2,500 J b. 4,000 J c. 20,000 J d. 160,000 J
2. A car exerts a force of 500 newtons to pull a boat 100 meters in 10 seconds. How much power does the car use? a W b W c. 50 W d W
3. In which of the following cases is work being done on an object? a. pushing against a locked door b. suspending a heavy weight with a strong chain c. pulling a trailer up a hill d. carrying a box down a corridor
1. A tractor exerts a force of 20,000 newtons to move a trailer 8 meters. How much work was done on the trailer? a. 2,500 J b. 4,000 J c. 20,000 J d. 160,000 J
2. A car exerts a force of 500 newtons to pull a boat 100 meters in 10 seconds. How much power does the car use? a W b W c. 50 W d W
3. In which of the following cases is work being done on an object? A. pushing against a locked door B. suspending a heavy weight with a strong chain C. pulling a trailer up a hill D. carrying a box down a corridor C is the only answer where Force and displacement are in the same direction!
1. Make and complete the Concept Map on pg Answer questions # Reviewing Content section (pg 441)