11/25/09
Oregon State University PH 201, Lecture #26
1
A 5.00 kg block is initially moving horizontally at 4.00 m/s across a
frictionless surface.
Then it slides down a frictionless ramp, losing
2.00 m of altitude in the process.
At the bottom of the ramp, it
encounters another horizontal surface, but this one exerts a kinetic
friction force of 10 N on the block.
How far across this surface will
the block slide before coming to rest?
1.
4.0 m
2.
5.8 m
3.
9.8 m
4.
13.8 m
5.
None of the above.

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11/25/09
Oregon State University PH 201, Lecture #26
2
Mechanical Power
Simply put,
power
is a measure of
the change or transfer of energy
per unit time
.
Of course, this is the most general de
fi
nition—it could
include any form of energy (heat, radiation, work, etc.).
For our
fi
rst
purposes, we look only at the rate at which work is done:
mechanical power
.
P
avg
=
W
/
t
In general,
W
=
Fx·
cos
±
, but note that when
F
is constant and in the
same direction as
x
,
W
=
Fx
.
In that case:
P
avg
=
Fx
/
t
=
Fv
avg
Example
:
What average mechanical power is required to accelerate a
1000 kg car from rest to 20 m/s in 10 seconds?

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