1. Work &Power

2. What is work ?
The product of the force applied to an object and the distance through which that force is applied.

3. Is work being done or not?
Mowing the lawn
Weight-lifting
Moving furniture up a flight of stairs
Pushing against a locked door
Swinging a golf club
YES NO

4. Calculating Work The force must act in the direction of the movement.
work = force x distance
The force must act in the direction of the movement.

5. Your Turn to Practice
A rock weighing 2 N was lifted 3 m. How much work was done?
A rock weighing 6.5 N was held 2 m above the ground. How much work was done?
It took 600 N of force to move a car 4 m. How much work was done?

6. Do you do more work when you finish a job quickly?
Work does NOT involve time, only force and distance.
No work is done when you stand in place holding an object.
Labeling work: w = F x d
Measured in joules (because it’s ENERGY!)

7. Power How quickly work is done. Amount of work done per unit time.
If two people mow two lawns of equal size and one does the job in half the time, who did more work?
Same work. Different power exerted.
POWER = WORK / TIME
Measured in Watts!

8. watts Used to measure power of light bulbs and small appliances
An electric bill is measured in kW/hrs.
1 kilowatt = 1000 W

9. Your Turn
If it takes you 10 s to do 150 J of work on a box to move it up a ramp, what is your power output?
A light blub is on for 12 s, and during that time it uses 1,200 J of electrical energy. What is the wattage (power) of the light bulb?

10. Machines A device that makes work easier.
A machine can change the size, the direction, or the distance over which a force acts.

11. Forces and Work involved:
Output Force (Work Output)
Force applied by a machine
Work done by a machine
Input Force (Work input)
Force applied to a machine
work done on a machine

12. Can you get more work out than you put in?
Work output can never be greater than work input.

13. Efficiency Efficiency can never be greater than 100 %. Why?
Some work is always needed to overcome friction.
It is a percentage comparison of work output to work input.
work output (WO) ÷ work input (WI)