**Second Examination
**

Physics 103

21 October 1980
#1.A box of mass **10.0 kg** is at rest on a horizontal floor, and the
coefficient of static friction is **µ**_{s} = 0.20. a force, **F**,
acts downward on the box at an angle of **30**^{o} to the horizontal,
as sketched. The value (**in Newtons**) which the magnitude of **F**
must exceed if the box is to be set in motion on the floor is most nearly equal
to:

**Answer:**

#2. The braking force required to stop an automobile weighing **3200 lb**
on a level road (**µ**_{k} = 0.3), assuming the drive
"jams" on the brakes and decelerates at **a = -10 ft/sec**^{2},
is

**Answer:**

#3. A block of mass ** m** is held at rest on a frictionless inclined plane
by means of a cord, as shown in the sketch. The cord is stretched parallel
to the plane under tension **T**, and the plane is inclined at an angle **q
**above the horizontal. If ** N
** is the normal force exerted by the
plane, the correct free-body-diagram for the block is

**Answer:**

#4. Mass **m**_{A} is connected to mass **m**_{B }by
a massless cord hung over a massless, frictionless pulley, as shown. The
kinetic coefficient of friction between **m**_{A }and the table is **µ**.
The mass **m**_{B} accelerates **downward**. The **positive**
acceleration of **m**_{B }is equal to

**Answer:**

#5. A conical pendulum rotates as shown with the string making an angle
of **37**^{o} with the vertical support. If the distance from
the ball to the support wall is **0.5 meter**, the linear speed of the ball
is about

**Answer:**

#6. A railroad flatcar loaded with crates has a coefficient of static
friction **µ**_{s} between the crates and the floor. If the
train is moving with speed **V**, the shortest distance in which the train
can be stopped without letting the crates slide is equal to

**Answer:**

#7. Two blocks are in contact with a frictionless, horizontal table. If **m**_{1}
= 3.0 kg and **m**_{2} = 5.0 kg, and a horizontal force of **2.0
Nt **acts on **m**_{1} (see sketch), the magnitude of the **force
of contact** between the two blocks is most nearly equal to

**Answer:**

#8. A block of mass **M = 15 kg** starts with a velocity of **v**_{0}
= 1 m/sec on a **30**^{o} incline at a distance of **1 meter**
above the ground. It slides down the rough incline and has a velocity **v**_{f}
= 3 m/sec when it reaches the ground. What is the c**oefficient of
kinetic friction**?

**Answer:**

#9. A ball dropped from height **h**_{1} with zero initial speed hits
the sidewalk and bounces straight back up to a lesser height **h**_{2}.
The **total work done** on the ball by air resistance and the sidewalk during
this process is
**Note**: The ball does not spin at any time.

**Answer:**

#10. The internal energy generated by a hockey puck (**m = 0.5 kg**) sliding on level ice
(**µ**_{k} = 0.10) for **70 meters** is
approximately

**Answer:**

#11. A block of mass **10 kg** compresses a light spring (**k = 1000 Nt/m**)
by a distance of **0.25 m** from its equilibrium position. Neglecting
friction, the **velocity of the block** when the spring reaches its
equilibrium position is most nearly equal to

**Answer:**

#12. The block in **Problem #11** then slides on a rough surface (**µ**_{k} =
0.15). The distance, **d**, it travels before coming to rest is
approximately

**Answer**

#13. A roller coaster starts approximately from rest at the top of a hill **90
feet** high. It goes on a smooth track through a loop of diameter **30
ft**. At the top of the loop, the roller coaster's velocity ( **in
ft/sec**) is most nearly equal to

**Answer**

14. A string has a length of **1.5 m**. A ball (**mass 2.0 kg**) is
attached at one end, and the other end is fixed to a frictionless pivot.
The ball is held so that it stretches the string horizontally, and it is then
released. (See sketch) When it reaches the lowest point on its
swing, its **speed** (**in m/sec**) is most nearly equal to

**Answer:**

#15. A man lifts **ten 7 kg** boxes **per minute** to a conveyer belt
which is **21.5 meters** off the ground. How much **power** does he
exert in doing this?

**Answer:**