Midterms Exam 1: Kinematics, dynamics, work and energy

Question 1

In the absence of an external force, a moving object will __________.

Answer 1

move with constant velocity

Question 2

A constant net force acts on an object. Describe the motion of the object.

Answer 2

the object experiences constant acceleration

Question 3

If you blow up a balloon then release it, air comes out of the balloon and the balloon will fly away. This motion is an example of which of Newton’s laws?

Answer 3

Newton’s Third Law: F1 = -F2

Question 4

Why does the Moon move more than the Earth in the Earth-Moon gravitational interaction, from the perspective of a distant observer located at the Sun?

Answer 4

The Moon has less mass than the Earth

Question 5

An object of mass m is hanging by a string from the ceiling of an elevator. The elevator has broken and is in free fall. What is the tension in the string?

Answer 5

T = mg

Question 6

An object of mass m is hanging by a string from the ceiling of an elevator. The elevator is moving upward but slowing down with an acceleration of magnitude less than g. What is the tension in the string?

Answer 6

T

Question 7

When an object is undergoing uniform circular motion, the direction of the net force on it is __________.

Answer 7

directed toward the center of the circular path

Question 8

If you push twice as hard against a stationary brick wall, the amount of macroscopic work that you do __________.

Answer 8

is zero because there is no displacement

Question 9

Suppose that an object travels from one point in space to another. Make a comparison between the displacement and the distance traveled.

Answer 9

The displacement is either less than or equal to the distance traveled.

Question 10

An object moving in the +x direction experiences an acceleration of 2.0 m/s^2. This means the object is __________.

Answer 10

increasing its velocity by 2.0 m/s in every second

Question 11

Objects A and B both start at rest. They both accelerate at the same constant rate. However, object A accelerates for twice the time as object B. What is the final displacement of object A compared to that of object B?

Answer 11

four times as far

Question 12

A ball is thrown straight up from the street in Richmond and reaches some height before falling back down. Which is true about the direction of the velocity and acceleration of the ball while it is going up?

Answer 12

Its velocity is upward, and its acceleration is downward.

Question 13

What is true about the terminal velocity of a falling object?

Answer 13

There is no net force on the object, drag forces must be present, the magnitude of the velocity decreases if the resistive force constant (b) increases, and the magnitude of the terminal velocity increases if the gravitational acceleration increases.

Question 14

An object is moving in a straight line with constant acceleration. Initially it is traveling at 16 m/s. Some time later it has moved 39 m and is traveling at 10 m/s. How much time later is this?

Answer 14

t = 3.0 s

Question 15

If a 1000-kg car starts from rest and has a force of 6000 N applied to accelerate it, what is its speed after 2.0 s and how far has it moved over those 2.0 s?

Answer 15

v = 12.0 m/s
x = 12.0 m

Question 16

A cannon ball is launched from a pirate ship at the level of the water with an initial velocity of 100.0 m/s at an angle of 60deg above the horizontal. What are the vertical and horizontal components of the initial velocity? Ignoring air resistance, how far away along the water does the projectile land?

Answer 16

v(x0) = 50.0 m/s
v(y0) = 86.6 m/s
x = 884 m

Question 17

A warehouse employee pulls a 50.0-kg box with a rope across a horizontal floor with a force of 100 N in a direction 15deg above the horizontal. The coefficient of kinetic friction is 0.2. What is the normal force on the box? What is the acceleration of the box and in what direction is it?

Answer 17

N = 464 N
a(x) = 0.076 m/s^2 in the +x direction

Question 18

Consider two masses hanging on opposite sides of a very flexible rope that passes over a pulley mounted to the ceiling. Assume that the mass of B is greater than the mass of A, and that the mass of the rope is negligible compared to the masses of A and B. What is the acceleration of B in terms of m(A), m(B), and g?

Answer 18

a(B) = g((mB-mA)/(mB+mA))

Question 19

An object slides across a horizontal surface in the +x direction. Because of friction, it slows and comes to a stop with an acceleration of -a. What is the coefficient of kinetic friction in terms of a and g?

Answer 19

µ = a/g

Question 20

An object of mass m slides down an inclined plan where the coefficient of kinetic friction is µ. The angle between the inclined surface and the horizontal is θ. What is the acceleration of the object down the plane in terms of g, θ, and µ?

Answer 20

a = g*(sinθ - µcosθ)

Question 21

The planet Mars goes around the Sun with a period of almost two Earth-years (687 days). What is the frequency of Mars’ orbit in Hz? Approximating Mars’ orbit as circular, with what speed does Mars travel in its orbit if the Mars-Sun distance is taken to be 2.3E11 m? What is the acceleration on Mars and in what direction is it?

Answer 21

f = 1.7E-8 Hz
v = 25000 m/s
a = 2.62E-3 m/s^2 toward the center of orbit (i.e. the Sun)

Question 22

A pool stick, angled 10deg below the horizontal, pushes a ball for a distance 2.0 cm along a horizontal pool table with a force of 5.0 N. Ignoring friction and the rotation of the ball, what is the ball’s final kinetic energy?

Answer 22

K = 0.098 J