Transcript WorkandPower - Cobb Learning

Work: applying a force that moves something
W = F•d•cosθ
Ignore the cos θ if
there’s no angle b/c
cos of 0˚ is 1
1.
A box weighing 50 N is held in the air by a bored
man. How much work does he do on the box?
2.
A 15 kg box is lifted up 1.5 m into the air by a
happy man. How much work does he do on the
box?
W = ______ unit: _____
F= _______ unit: _____
d= ___________ unit _____
Displacement, remember, is slightly different from distance.
Displacement includes direction – how far you get. Distance has
no direction – how far your feet walk total.
The force must be in the direction of the displacement.
•Pushing a box to the right 5 m is work,
•Pushing down on a toy car driving to the right is not
work.
•Pushing against a wall requires a _____ but is not
actually _____.
Power: how fast you do work. “The rate of work.”
P = W = F•d
t
t
P= ______ unit: _____ or ______
W= ______ unit ______
t= ______ unit: ___
Work
Vs.
Power
Can a weak person do the same amount of work as a
strong person? _____.
Can a weak person do the work in the same time as
the strong person? _____, because the strong person
has more _______.
A man weighing 735 N runs up a 60 m tall
hill, and gets to the top in 15 s. How much work did he
do? How much power did it take?
Who does more work, Strong Stan, who carries 500
books across the room in 5 minutes, or Weak Will,
who does the same job in 30 minutes? What is
different between them?
A woman pushes a 50 N box across the floor at
constant speed a displacement of 5 m, by applying a
force of 15 N. How much work does she do on the
box?
A child pulls a box across the floor at a constant speed
a displacement of 10 m, by pulling on a rope with a
force of 20 N that is at an angle of 15 degrees to the
floor. How much work does the child do on the box?
Haha! The past! What is the coefficient of friction
between the box and the floor?
An 80 kg athlete pulls himself up a 6 m rope in 20
seconds. How much work did he do?
What was the power required for this feat?
A woman pulls a 75 N box along the floor at a constant
velocity a displacement of 15 m in 8.5 seconds. If she
does this by applying a force of 33 N along a rope at a
35 degree angle, what power does she exert?