Physics 4A
SECOND EXAM
Chapters 1  8
Fall 1998
1
Name:_________________________________________
Solve the following problems in the space provided.
Use the back of the page if needed.
Each
problem is worth 20 points.
You must show your work in a logical fashion starting with the
correctly applied physical principles which are on the last page.
Your score will be maximized if
your work is easy to follow because partial credit will be awarded.
1.
A 50.0kg crate skids down a 30.0˚ ramp.
The coefficient of friction is 0.450.
Find the
acceleration.
The three forces on the crate are gravity, normal force and friction.
Choose the acceleration along the x axis.
Using the free body diagram
carefully apply the Second Law to each direction separately,
Σ
F
x
=
ma
x
⇒
F
g
sin
θ 
F
fr
=
ma
⇒
mgsin
θ 
F
fr
=
ma
(1),
Σ
F
y
=
ma
y
⇒
F
n

F
g
cos
θ =
0
⇒
F
n
=
mgcos
θ
(2),
where the mass/weight rule has been used.
Using the definition of coefficient of friction and eq. 2,
μ≡
F
fr
F
n
⇒
F
fr
=μ
F
n
=μ
mg
cos
(3).
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 Fall '09
 Kagan
 Energy, Force, Friction, Potential Energy, Work, Law of Conservation of Energy

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