1. ENERGY, WORK AND
SIMPLE MACHINES
CHAPTER 10

2. ENERGY AND WORK
• How do we define the relationship between work
and energy?
• How can we calculate work done?
• How do we calculate power used?

3. WORK EQUATION
•

4. WORK EQUATION
•

5. WORK EQUATION
• Right side is Fd
• F = force (in Newtons)
• d = distance (in meters)
• We define:
• W = Fd
• Work = forces times distance

6. WORK ENERGY THEOREM
•

7. EXAMPLE
• A 105 g hockey puck
is sliding across the
ice. A player exerts a
constant 4.50 newton
force over a distance
of 0.150 m. How
much work does the
player do on the
puck? What is the
change in energy?

8. LIFTING A BOOK
• When is the work positive?
• When is the work negative?
• When is the work zero?

9. WORK AGAINST GRAVITY
• W = Fd
• The work of lifting something is equal to the weight
of the object times the distance lifted
• Weight =
• So W =

10. WORK
• Since work equals the change in KE, the unit is the
same
• Work is measured in joules
• One joule happens when a force of 1 N acts for 1 m
• An apple is approximately a newton, so lifting an
apple 1 meter is about 1 Joule of work

11. WORK
• What if our force is not applied in a straight line?
• Will it be as effective?
• How do we account for this?

12. WORK
• W = Fdcosɵ
• ɵ is between the
force and the
direction of
displacement
• If he pushes the car
10.0 m, how much
work did the man
do?

13. WHAT TO INCLUDE IN WORK
• Which direction do
the normal force
and gravity point?
• ɵ is …
• What about
friction?

14. EXAMPLE
• A sailor pulls a boat
a distance of 30.0 m
along a dock using
a rope that makes
a 25.0° angle with
the horizontal. How
much work does
the sailor do on the
boat if he exerts a
force of 255 N on
the rope?

15. TRIG REFRESHER
• SOH-CAH-TOA

16. WORK AGAINST GRAVITY
• Pushing up a ramp,
walking up stairs
• What do we use for
d?

17. HOMEWORK
• Page 287, # 1 – 3
• Page 291, # 4 - 8

18. GRAPHICAL METHOD
• Area under force vs
displacement curve
is work
• How much work?

19. GRAPHICAL METHOD
• Force exerted by a
spring
• Work =
• Area of a
trapezoid=
• ½ h (b1 + b2)

20. MULTIPLE FORCES
• If several forces are exerted on a system, calculate
the work done by each force, then add the results

21. POWER
•

22. POWER
• Three student going
up stairs
• If they started at the
same time…
• How does their work
compare?
• How does their power
compare?

23. POWER
• On a ten-speed
bike, there is a
combination of
force and speed
that will produce
the maximum
power

24. EXAMPLE
• An electric motor lifts an elevator 9.00 m in 15.0 s by
exerting an upward force of 1.20 x 104 N. What
power does the motor produce in kW?

25. HOMEWORK
• Page 264, # 9 – 14
• Page 265, # 15 - 21