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Intro Physics
Test #3 - sections 1- 2
28 October 2013
GENERAL INSTRUCTIONS: Please write your name on the back of the last page. Show all work
except arithmetic. When solving problems I require starting from the simplest expressions and
continuing from there. Drawing a picture is always helpful. If anything is unclear ask me to clarify.
SECTION I: Answer all questions each worth four points. For true/false and multiple-choice
questions it is REQUIRED for you to write the answer in the left margin.
1. (Yes / No) Is it possible for an object to change speed if the net force acting on it is zero?
(Yes / No) Is it possible for an object to change direction if the net force acting on it is zero?
2. Like Garfield, consider Newton’s first law of motion and complete this comic:
3. Newton's Third Law of Motion is
a) a special case of Newton's First Law of Motion.
b) a special case of Newton's Second Law of Motion.
c) independent of Newton's other Laws of Motion.
d) applicable only when air resistance can be neglected.
4. A satellite orbits at an altitude equal to 50% the radius of earth. What is the acceleration due to
Earth’s gravitational force there?
a) 14.7 m/s2
b) 9.80 m/s2
c) 6.53 m/s2
d) 4.36 m/s2
e) 2.45 m/s2
f) 0 m/s2
5. (T/F) “Universal gravitational constant” is another way of saying “acceleration due to gravity.”
6. Give an example (in words not diagrams) of an object that touches a surface where the directions of the gravitational force and the normal force are a) Not collinear and b) Perpendicular.
7. A popular activity on airplane flights with Govies is to calculate the acceleration of the plane by
dangling keys from a shoelace. Yes CLM we’re doing it this year too. It is easy to derive an
expression for acceleration assuming it is constant. Describe how you would verify or refute the
assertion of constant acceleration.
8. I am often amazed that good students who know physics fail to wear a helmet while riding a
bicycle. For each of Newton’s laws of motion (1st, 2nd, and 3rd, separately) describe how that
particular law shows that wearing a helmet is an obvious safety practice. SCM will follow this.
Section II: Complete problem 9 worth 10 points. Work on this paper.
9. A small block of mass m1 sits on top of a larger block of mass m2, which is initially at rest on a
table. A horizontal force is applied to the larger mass and both blocks accelerate to the right.
Friction is present between the larger mass and the table and between the two blocks. Draw two
separate Free Body Diagrams, one of each block. As always, label all the forces acting on the
object and clearly indicate the direction of these forces.
m1
FP
m2
Section III: Complete 3 of the following 4 problems, each worth 20 points. You may work on this
paper. If you get stuck on one part(s), but you know how to do the other part(s) write down your
first few steps so you can earn partial credit. Cross out the problem you chose not to be graded.
10. A rope at a 60.0° angle pulls a 5.00-kg block, resting on a level surface, with a constant 40.0 N
force. Additionally, it is subjected to a kinetic frictional force.
a) Draw a Free Body Diagram for this.
b) Compute the magnitude of the normal force.
c) If the block’s acceleration is measured to be +3.00 m/s2 (along the horizontal), compute the
frictional force (just the force not the coefficient of friction).
11. SMH was very brave, willing to lie down while I rotated a bucket of water in a vertical plane
directly over her. If the distance from my shoulder to the water is 90.0 cm, using an FBD and
Newton’s Laws compute the maximum time period for one rotation of the bucket such that no
water spills out.
12. In the experiment concerning friction (#10) one method for determining the coefficient of
kinetic friction (µk) was to slide a block of mass m, without accelerating it, up a ramp inclined at
angle 𝜃 by applying a constant tension force FT parallel to the incline which you measured with
the LabQuest.
a) Using a Free Body Diagram and Newton’s laws find an expression for µk in terms of these
variables (𝑚, 𝜃, 𝐹𝑇 ) and g, the local acceleration due to gravity.
b) Compute the value for a 90.0-gram mass on a 16.0º incline pulled with an average force of
0.478 N, as measured by DCF and ASH.
13. The moon (mass m) orbits the Earth (mass M) in a period T with a circular orbit or radius r.
This radius is measured properly from the center of the moon to the center of the Earth.
a) Given the radius of the Earth is R, derive an expression for the density of the Earth in terms
of these variables and G. Assume the Earth is a perfect sphere. Recall the volume of a sphere
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is 𝑉 = 3 𝜋𝑅 3 and that density is given by  = mass/volume. You should get
𝜌=
3𝜋 𝑟 3
( ) .
𝐺𝑇 2 𝑅
If you did not get part (a) you can still complete part (b). 
b) Compute the average density of the Earth given r = 3.84 × 108 m, R = 6.38 × 106 m,
T = 27.3 days, and G = 6.67 × 10–11 Nm2/kg2.