PHYS 211 Recitation Review Problems: Solutions

Physics 211 Fall 2012 Midterm 2 Recitation Review Solutions
5. In class we watched a video of a hammer and feather dropped on the moon. Both stared from
the same height and landed on the lunar surface. Assume that the mass of the hammer is 100
times greater than the mass of the feather.
During the time that they fall freely, consider the following two statements:
i) The net force on the hammer is equal to the net force on the feather.
ii) The net acceleration of the hammer is equal to the net acceleration on the feather.
A) Without knowing more numbers, we cannot decide
B) Both statements are true
C) i is true, but ii is false.
D) i is false, but ii is true
E) Both statements are false.
The forces (mg) are DIFFERENT because of the different masses. However the acceleration of
both is the same, namely g moon .
6. A rover is moving about on a flat Martian surface. Its position as a function of time is given
m 2
m
x t 3 t , y t 5 t, z t 0
extremely precisely by 2
s
(You should assume there is rolling friction for the rover on Mars!)
What can you conclude about the net force on the rover?
s
A) There is not enough information given to conclude anything precise about the net force.
B) The magnitude of the net force is a constant in time, and always points in the + ˆ i direction.
C) The magnitude of the net force is not constant in time, but always points in the + ˆ i direction.
D) The magnitude of the net force is a constant in time, but the direction changes with time.
E) The magnitude of the net force is not constant in time, and the direction also changes with
time.
Find the acceleration by taking 2 time derivatives. Only a
x (t) = 6 m/s 2 is non-zero. Because the
acceleration is constant in the +i direction, so is the net force.
7. A penny sits on an old fashioned (flat, horizontal) circular record turntable,
rotating at a constant speed as illustrated in the "top-down" diagram at the right.
1¢
Which of the following sets of vectors best describes the directions of the velocity,
acceleration, and net force acting on the penny at the point indicated in the
diagram? (Note: this is not a force diagram, I'm just asking for the direction of
these three different vectors)
F
v
a
F
a=0
v
F
a=0
v
F
a
v
F
a
v
A)
B) C)
D) E)
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