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?
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?
