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HOWARD UNIVERSITY
PHYS 001RECITATON ASSIGNMENT # 7
Part I. Work and Energy Practice Problems
1)
Find the work done by a force of 300.0 N exerted on an object to lift it vertically to a
distance of 240.0 cm above the ground.
Answer:
The work done on the object is
W= F cos
θ
s=(300.0 N) (cos 0
°
) (2.4 m)=
7.2 x 10
2
J.
2)
An object of mass 450 g moves along a horizontal surface with constant velocity due to
a force that pulls the object (traction force). Find the work done by the net force and the
force of friction over a distance of 2 m if the coefficient of kinetic friction is 0.6.
Answer:
The work done by the net force is
W=F
net
cos
θ
s= (0 N) cos 0
°
(2m)=
0 J.
The work done by the force of friction is
W=F
frict
cos
θ
s=
μ
k
F
N
cos
θ
s=(0.6)(0.450 kg)(9.8 m/s
2
) cos 180
°
(2m)=
5.292 J.
3)
In practice problem 3, find the work done by the weight, and the work done by the force
of friction.
Answer:
The work done by the weight is
W=F
g
cos
θ
s= (0.450 kg)(9.8 m/s
2
)cos 90
°
(2 m)=
0 J.
The work done by the force of friction is
W=F
frict
cos
θ
s=
μ
k
F
N
cos
θ
s=(0.6)(0.450 kg)(9.8 m/s
2
) cos 180
°
(2m)=
5.292 J.
4)
At a construction site, a plank of mass 425 kg is lifted to a height of 11.2m above the
ground by a constant force in 1.00 minute. The plank is lifted at a steady pace. Find the
power needed to produce this displacement.
1

P a g e
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View Full DocumentAnswer:
The power needed to produce this displacement is
W
s
J
s
m
s
m
kg
F
t
W
P
g
5
.
777
60
46648
60
)
2
.
11
(
0
cos
)
/
8
.
9
)(
425
(
cos
2
=
=
°
=
=
=
t
s
θ
.
5)
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 Fall '10
 Staff
 Physics, Energy, Force, Work

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