PHY 1151
Fall 2002
Second Hour Exam
October 23, 2002
1. A particular roller coaster has a loop with a radius of 20 m. What is the minimum speed the coaster can have at the top of this loop so that passengers just loose contact with the seat?
2. In loading a certain dart gun, a 50 g (0.050 kg) dart is pushed
in 12 cm (0.12 m) before the gun is cocked. A force of 20 N is required at the
end.
Find the following:
a) the spring constant k of the spring.
b) the total amount of energy stored in the spring.
c) the speed of the dart when it leaves the gun.
3. A 600 g glider moves on an air track with a velocity of 10 cm/s
to the right while a 400 g glider moves to the left with a velocity of 20 cm/s.
The two collide totally inelastically.
What is their common velocity after the collison?
4. A uniform pole 6 m long weighs 300 N and is attached by a pivot at one end
to a wall. The pole is held at an angle ¯of 30° above the horizontal
by a horizontal guy wire attached to the pole 4.0 m from the end attached to
the wall. A load of 600 N hangs from the upper end of the pole.
Find the following:
a) the tension in the guy wire and
b) the components of the force exerted on the pole by the wall.
5. Concept Questions:
i) Consider a car that travels along a circular track at a constant speed. What
do you know about the net force on the car?
The net force points toward the center of the circle.
The net force is often called the "centripetal force", Fc .
Fc = m v2 / r
ii) Jupiter’s mass is 300 times greater than Earth’s mass. Yet you
would only weigh 3 times as much if you were on Jupiter’s surface compared
to your weight here on Earth’s surface. Why is that?
F = G M m / r2
While MJ = 300 ME, the radius for Jupiter is about ten times the radius for Earth; that is, rJ = 10 rE.
While the much greater mass tends to make the force of gravity
larger, the larger radius tends to make the force of gravity smaller.
iii) What does it mean to say a force is a "non-conservative force"?
And give a common example of "non-conservative force".
For a conservative force, we can construct a potential energy, PE, and use
that in applying the conservation of energy. For a non-conservative force, no
potential energy, PE, can be constructed. A good and common example is the force
of FRICTION.
iv) Distinguish between momentum and kinetic energy. Or, what is the difference
between momentum and kinetic energy? Or, compare and constrast momentum and
kinetic energy.
Momentum is a vector and kinetic energy is a scalar. There is
no direction associated with kinetic energy. Kinetic energy is always non-negative
(KE is either zero or positive) while momentum, being a vector, can have a positive
or negative value for any of its components.
Momentum and kinetic energy both depend upon the mass and velocity (or speed)
of an object.
v) What are the two conditions of equilibrium?
1) The sum of all the forces on an object in equilibrium must be zero.
2) The sum of all the torques on an object in equilibrium must be zero.
(or -- The sum of all the clockwise torques must equal the sum of all the counterclockwise torques).