PHY 121 - Quiz 1 (Solution)

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Duration: 10 mins
NAME:
PHY 121 - Quiz 1 (Solution)
Q1: The pendulum shown swings between −π/4 and +π/4. Ignore friction and assume that the string is massless.
The eight numbered arrows represent directions to be referred to when answering the following questions (3 points).
FIG. 1: Use the numbered arrows to answer the questions.
A. Which of the following is a true statement about the acceleration ⃗a of the pendulum bob
○ ⃗a is equal to the acceleration due to gravity.
○⊙ ⃗a is equal to the instantaneous rate of change in velocity.
○ ⃗a is perpendicular to the bob’s trajectory.
○ ⃗a is tangent to the bob’s trajectory.
Reasoning: Acceleration is defined as the instantaneous rate of change in velocity.
B. What is the direction of ⃗a when the pendulum is at position 1? Write the letter of the arrow parallel to the direction.
○ H Reasoning: Acceleration has a component gsin(θ) in the specified direction
C. What is the direction of ⃗a when the pendulum passes position 2? Give the letter of the arrow that best
approximates the direction.
○ C Reasoning: The tension in the string provides the acceleration equal in magnitude but
opposite in direction to accleration due to gravity ⃗g
D. What is the direction of ⃗a when the pendulum reaches position 3? Give the letter of the arrow that best
approximates the direction.
○ F Reasoning: Acceleration has a component gsin(θ) in the specified direction
E. At which position does the pendulum have the largest positive angular acceleration about the pivot point? The
angular acceleration is the rate of change of the angular velocity of the string. Give the number of the position, and
take positive angular acceleration to be counterclockwise.
○ 1 Reasoning: At position 2, the angular speed of the pendulum is constant, so the angular
acceleration is zero. At position 3, the angular acceleration is negative.
F. At which position or positions is the acceleration vector ⃗a almost parallel to the velocity vector ⃗v.
○ position 2 only ○ positions 1 and 2 ○ positions 2 and 3 ○⊙ positions 1 and 3
Reasoning: Since the value of instantaneous velocity at the ends point is zero.

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Q2: Understanding Newtons Laws (3 points)
A. An object cannot remain at rest unless which of the following holds?
○⊙ The net force acting on it is zero.
○ There are no forces at all acting on it.
○ The net force acting on it is constant.
○ There is only one force acting on it.
Reasoning: Newtons first law: A body will remain in a state of rest or uniform motion unless acted upon by
external unbalanced force.
B. If a block is moving to the left at a constant velocity, what can one conclude?
○ There is exactly one force applied to the block.
○ The net force applied to the block is directed to the left.
○⊙ The net force applied to the block is zero.
○ There must be no forces at all applied to the block.
Reasoning: Same as above.
C. A block of mass 2 Kg is acted upon by two forces: 3 N(directed to the left) and 4 N (directed to the right). What
can you say about the block’s motion?
○ It must be moving to the left.
○ It must be at rest.
○ It must be moving to the right.
○⊙ It could be moving to the left or right or at rest.
Reasoning: It could be moving to the left or right or at rest.
D. A massive block is being pulled along a horizontal frictionless surface by a constant horizontal force. The block
will —
○ continuously change direction.
○ move with constant velocity.
○⊙ move with a constant nonzero acceleration.
○ move with continuously increasing acceleration.
Reasoning: Newtons second law: Force(F) ∝ Rate of chance of momentum (m a)
E. Two forces, of magnitude 4 N and 10 N are applied to an object. The relative direction of the forces is unknown.
The net force acting on the object:
Circle all possible correct answers
○1.
cannot be equal to 5 N 2. cannot be equal to 10 N
3. must be greater than 10 N 4. cannot be directed the same way as the force of 10 N
Reasoning: Since force is a vector quantity deduce the smallest possible force and the largest force using different
combination for orientation of the force vectors. Smallest, when the two forces are in opposite direction and largest,
when they are in the same direction.