Exam B: Newton’s Laws
(Total points=100)
Name:
Signed:
PHY 161 General Physics I: Mechanics and Thermodynamics
Physics Department — Mercer University — Spring 2010
• Every student should do parts A (multiple choice), B and C.
• However, you should only do either D or E.
I.e., you may work both D and E, and both will be graded, but you will only get credit for the
one with the higher grade.
• Except for multiple choice questions, answers provided without explanation will receive little or no credit,
so you must SHOW ALL YOUR WORK.
• All answers should be decimal numbers. Please do not leave answers in terms of π, fractions, square roots,
trig functions, or inverse trig functions.
• Except as noted, provide numerical answers that are either EXACT, or accurate to THREE SIGNIFICANT
FIGURES, or WITHIN 1% of the exact answer.
• Acceleration due to gravity at the Earth’s surface: g = 9.81m/s2 .
• For all problems in this exam, ignore the effects of air resistance.
A:(35 points total, 5 points @) Multiple Choice
1. A bell hangs from a blue rope. A red rope attached to the side of the bell is pulled in the horizontal
direction. This causes the blue rope to make an angle less than 90º with the vertical. Which of the
following statements is true?
A) The red rope is under greater tension than the blue rope.
B) The red rope is under the same tension as the blue rope.
C) The red rope is under smaller tension than the blue rope.
D) The tensions cannot be compared unless the mass of the bell is known.
E) The tensions cannot be compared unless the angle between the blue rope and the vertical is known.
2. Two identical boxes are sliding across the same floor at the same speed. The first box contains a 20kg mass and the second box contains a 10-kg mass. Which box will the kinetic friction force stop first?
A) The box containing 20.0 kg will stop first.
B) Both boxes will stop at the same time.
C) The box containing 10.0 kg will stop first.
D) More information is needed to answer this question.
E) The answer depends on the magnitude of the coefficient of kinetic friction.
3. A person riding a Ferris wheel is strapped into her seat by a seat belt. The wheel is spun so that the
centripetal acceleration is g. Select the correct combination of forces that act on her when she is at the
top. In the table, Fg = force of gravity, down; Fb = seat belt force, down; and Fs = seat force, up.
A) Fg = mg, Fb = 0, Fs = 0
B) Fg = 0, Fb = mg, Fs = 0
C) Fg = mg, Fb = mg, Fs = 0
D) Fg = 0, Fb = 0, Fs = mg
E) Fg = mg, Fb = 0, Fs = mg
4. If a satellite moves above the Earth's atmosphere in a circular orbit with constant speed, then
A) its acceleration and velocity are in the same direction.
B) the net force on it is zero.
C) its velocity is constant.
D) it will fall back to Earth when its fuel is used up.
E) its acceleration is toward the Earth.
5. Which of the following free-body diagrams best represents the free-body diagram, with correct
relative force magnitudes, of a person in an elevator that is traveling upward with a downward
acceleration less than g? Ff is the force of the floor on the person and Fg is the force of gravity on the
person.
A)
B)
C)
D)
E)
6. A professor holds an eraser at rest against a vertical chalkboard by pushing horizontally on it. He
pushes with a force that is much greater than is required to hold the eraser. The force of friction exerted
by the board on the eraser increases if he
A) pushes with slightly greater force in the same direction.
B) pushes with slightly less force in the same direction.
C) stops pushing.
D) pushes so his force is slightly downward but has the same magnitude.
E) pushes so his force is slightly upward but has the same magnitude.
7. A box with a weight of 45 N rests on a horizontal surface. A person pulls horizontally on it with a force of 10
N and it does not move. To start it moving, a second person pulls vertically upward on the box. If the
coefficient of static friction is 0.4, what is the smallest vertical force for which the box moves?
A)
B)
C)
D)
E)
18 N
20 N
25 N
45 N
70 N