mastering physics- newton's second law lab

4/7/13 Lab4 (Newton's second law)

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Lab4 (Newton's second law)

Due: 12:44pm on Saturday, February 9, 2013

Note: To understand how points are awarded, read your instructor's Grading Policy.

Newton's Second Law Tutorial

Work through the tutorial by clicking the image or link below, then answer the questions below.Newton's Second Law Tutorial

Part A

Object A has three times the mass of object B. Identical forces are exerted on the two objects. Whichstatement is true?

Hint 1. Newton’s Second Law

Use Newton’s second law, F = ma, to determine how the acceleration depends on the object’smass.

ANSWER:

Correct

Since F = ma, the acceleration is given by a = F/m, so the object with the lower mass has a higheracceleration.

PHY110ASPRING2013

Lab4 (Newton's second law) Resources

The speeds of the two objects are equal after 2 seconds.

The acceleration of object B is three times that of object A.

The accelerations of the two objects are equal.

The speed of object A is three times that of object B after 3 seconds.

Signed in as Ellie Kamens Help Close



Part B

A parachutist is falling toward the ground. The downward force of gravity is exactly equal to the upwardforce of air resistance. Which statement is true?

Hint 1. Relationship between acceleration and net force

What is the net force on the parachutist? Is the resulting acceleration upward, zero, or downward?

ANSWER:

Correct

Since the net force is zero, the parachutist’s acceleration is zero, so the velocity is constant in time.

Part C

A kilogram is a measure of an object’s __________.

ANSWER:

Correct

An object far from any source of gravity still has mass, even if no forces of gravity are acting on it.

Part D

A person who weighs 800 N (about 180 pounds) is standing on a scale inside an elevator. The elevator ismoving upward yet slowing down. The reading on the scale is __________.

Hint 1. Determining the acceleration and direction of the net force

What direction is the person’s acceleration? Remember that the net force (due to gravity pullingdown and the scale pushing up) is in the same direction as the acceleration.

The velocity of the parachutist must be zero.

The velocity of the parachutist is increasing with time.

The velocity of the parachutist is not changing with time.

The velocity of the parachutist is decreasing with time.

mass

weight

gravity

force



ANSWER:

Correct

Since the scale is pushing upward with a strength less than the force of gravity, the net force isdirected downward, causing the person to decelerate.

Part E

A person who weighs 800 N is squatting on a scale. Suddenly he starts jumping, extending his legs asquickly as possible, so that he can jump into the air. As the person is in the process of jumping(accelerating upward) the reading of the scale is ____________.

Hint 1. Direction of net force

In what direction is the net force acting on the person?

ANSWER:

Correct

Since the person is accelerating upward, the scale must be pushing upward more strongly than theforce of gravity.

Part F

A bowling ball and a small marble will fall downward to the surface of the Moon at the same rate. Why?

Hint 1. Newton’s Second Law

Use Newton’s second law, F = ma, to consider how the acceleration of a falling object depends onthe force of gravity and mass.

ANSWER:

less than 800 N

greater than 800 N

equal to 800 N

greater than 800 N

equal to 800 N

less than 800 N



Correct

The acceleration is given by a = F/m, so all objects fall at the same rate since the force of gravity isproportional to the object’s mass.

Part G

Two identical twins are falling toward the ground. The twin in the green jumpsuit is going down head first,and the twin in the red jumpsuit is going down belly first. The two twins have the same mass. Which twinreaches the fastest terminal velocity?

Hint 1. Terminal Velocity

The terminal velocity occurs when the upward force of wind resistance equals (in strength) thedownward force of gravity. How does wind resistance depend on the surface area of the fallingobject?

ANSWER:

Correct

Since the twin in the green jumpsuit (going head first) has a smaller surface area plowing through theair, she must fall faster in order for the force of air resistance to have the same strength as the force ofgravity.

Part H

Two parachutists have identical parachutes open and are falling toward the ground. The parachutist withthe red jumpsuit is much more massive than the parachutist with the blue jumpsuit. How do the terminalvelocities of the two parachutists compare?

Hint 1. Terminal Velocity

The terminal velocity occurs when the force of wind resistance balances the force of gravity.

ANSWER:

The force of gravity on an object in a vacuum is zero.

The ratio of the force of gravity exerted on an object to the object’s mass is the same.

The force of gravity is the same for each object.

the twin in the red jumpsuit

the twin in the green jumpsuit

Both twins have the same terminal velocity.



Correct

Since the red parachutist is more massive, he experiences a stronger force of gravity actingdownward. In order for the wind resistance to balance this stronger force, he must have a fasterterminal velocity.

Video Tutor: Tension in String between Hanging Weights

First, launch the video below. You will be asked to use your knowledge of physics to predict the outcome of anexperiment. Then, close the video window and answer the question at right. You can watch the video again atany point.

Part A

Consider the video tutorial you just watched. Suppose that we duplicate this experimental setup in anelevator. What will the spring scale read if the elevator is moving upward at constant speed?

Hint 1. How to approach the problem

What does the phrase "at constant speed" imply about the acceleration of the system?

ANSWER:

The blue parachutist has a higher terminal velocity.

Both parachutists have the same terminal velocity.

The red parachutist has a higher terminal velocity.

0

Less than 18 but greater than 0

18

More than 18



Correct

Since the elevator is not accelerating, the reading on the scale is the same as in the video.

Video Tutor: Chin Basher?

First, launch the video below. You will be asked to use your knowledge of physics to predict the outcome of anexperiment. Then, close the video window and answer the question at right. You can watch the video again atany point.

Part A

Suppose our experimenter repeats his experiment on a planet more massive than Earth, where theacceleration due to gravity is . When he releases the ball from chin height without giving it a

push, how will the ball's behavior differ from its behavior on Earth? Ignore friction and air resistance. (Selectall that apply.)

Hint 1. How to approach the problem

To answer this question, you must evaluate how a change in affects the pendulum’s trajectory

(choices A and B), period (choices C and D), and mass (choice E).

Trajectory: Here, the key is energy conservation. Think about how kinetic and potential energyinterchange as a pendulum swings. For the pendulum to return to a position higher than its startingpoint, the system of the pendulum and planet would have to gain mechanical energy. Where wouldthat energy come from? (The same argument applies to choice B.)

Period: You can answer this question by seeing whether the equation for the period of a pendulumincludes . Or you can use reasoning. A greater downward acceleration will cause the pendulum

to reach the bottom of its swing more quickly. What does that say about the pendulum’s totalperiod?

Mass: The pendulum will weigh more on the massive planet. Will it have more mass?

ANSWER:



Correct

The pendulum will swing back and forth more quickly (with a shorter period) because it is oscillating ina stronger gravitational field than that on Earth.

Video: Friction

Watch the video and then answer the following questions.

Part A

After he gets the block to move, what does Dr. Hewitt do to drag the block across the table at a steadyrate?

ANSWER:

It will take more time to return to the point from which it was released.

Its mass will be greater.

It will stop well short of his face.

It will smash his face.

It will take less time to return to the point from which it was released.



Part B

If Dr. Hewitt applies 3 N of force to keep the block moving at a steady rate, what must be the force offriction, and why?

Hint 1.

If the block is moving at a steady rate, what is its acceleration? Once you have figured out itsacceleration, what is the net force acting on it, based on Newton’s second law?

ANSWER:

Correct

Part C

You are dragging a block on a surface with friction at a steady speed of 2 m/s and exert a force of 5 N todo so. What is the force of friction? Why?

Hint 1.

If the block is moving at a steady rate, what is its acceleration? Once you have figured out itsacceleration, what is the net force acting on it, based on Newton’s second law?

ANSWER:

He applies a steadily increasing force.

He continues to apply a steady force.

He applies a steadily decreasing force.

Equal to 3 N, because the force of friction should exactly balance out the force Dr. Hewittapplies, so that there is no net force acting on the block.

More than 3 N, because the force of friction is what prevents the block from accelerating due theforce applied by Dr. Hewitt. So, it should overcome the force applied by Dr. Hewitt.

Less than 3 N, because otherwise the block would not move forward. Dr. Hewitt applies a forceof 3 N, so his force must overcome the force of friction.

Correct



Correct

Part D

What general rule can you conclude about the force needed to keep an object in motion at a steady rate?

Hint 1.

If an object is moving at a steady rate, what is its acceleration? Knowing the acceleration andNewton’s second law, can you determine the net force acting on the object? If you know the netforce and the force of resistance, what can you say about the force that you need to apply to keepthe object in motion?

ANSWER:

Correct

Score Summary:

Your score on this assignment is 66.7%.You received 2.67 out of a possible total of 4 points.

Equal to 5 N, because the force of friction should exactly balance out the force Dr. Hewittapplies, so that there is no net force acting on the block.

Less than 5 N, because otherwise the block would not move forward. You apply a force of 5 N,so this force must overcome the force of friction.

10 N, because you are exerting a force of 5 N at a speed of 2 m/s. Multiplying the two numbersgives you 10 N.

The force that you need to apply to keep an object moving at a steady rate is greater than theforce resisting the motion of the object.

The force that you need to apply to keep an object moving at a steady rate is less than the forceresisting the motion of the object.

The force that you need to apply to keep an object moving at a steady rate is equal to the forceresisting the motion of the object.

mastering physics- newton's second law lab

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