Chapter 5 Review

Video Review

Key Concept Summary

TA Summary

True/False

If an object is at rest, no forces are acting on it.

True False

Analysis

A ball is thrown straight up. Disregarding effects from the air, the forces acting on the ball from the time it leaves his hand until it returns to the ground are:

Its weight downward along with a steadily decreasing upward force. A steadily decreasing upward force from the moment it leaves the hand until it reaches its highest point beyond which there is a steadily increasing downward force of gravity as the object gets closer to the earth. A constant downward force of gravity along with an upward force that steadily decreases until the ball reaches its highest point, after which there is only the constant downward force of gravity. Only a constant downward force of gravity.

A beach ball rolls at constant speed across a smooth table top. Which of the following forces act on the ball as it rolls? (Select all correct answers.)

There are no forces because it is at constant speed. A downward force due to gravity. An upward force from the table. A net forward force from inertia.

A ball is thrown horizontally. While it is in the air after being thrown, which of the following forces act on the ball? (Select all correct answers.)

There are no forces because the person who threw it is no longer touching it. A downward force of gravity An upward force from inertia A forward force from the throw A contact force from the air

Two masses are suspended, one on each side, from a pulley as shown. What direction is the force the rope exerts on the pulley? A rope is draped over a pulley with a 5kg mass attached to the left side of the rop and a 1kg mass attached to the right side of the rope.

A rope is draped over a pulley with a 5kg mass attached to the left side of the rop and a 1kg mass attached to the right side of the rope.

Up on both sides Down on both sides Up on the side with the smaller mass, and down on the side with the heavier mass. Up on the side with the heavier mass, and down on the side with the lighter mass

When traffic lights are suspended from a cable stretched across the street, the cables holding traffic lights sags. If enough tension were applied to the cable, could the cable ever be made perfectly straight?

Yes No It depends on the composition of the cables.

What direction does friction point?

In the direction of motion. Opposite the direction of motion. Perpendicular to the direction of motion. Any of the above could be correct.

A book is at rest on a table on the Earth. Which which of the following forces must be acting on the book? (Select all correct answers.)

A contact force from the table A frictional force A contact force from the air Weight There are no force because the book is at rest.

You slam on the brakes and your car starts to slow down. What direction is the net force exerted on the car as it slows?

Forward Backward There is no net force because the car stops.

A boat is headed across a river. The motor on the boat exerts a large force due north. The river current exerts a small force due east. The net force on the boat most nearly points which direction?

Due North North-Northeast Northeast East-Northeast Due East

In an accident, a car collides with a semi-truck. Which of the following is true?

The force that the car exerts on the truck is greater than the force the truck exerts on the car. The force that the car exerts on the truck is the same as the force the truck exerts on the car. The force that the car exerts on the truck is less than the force the truck exerts on the car.

A sparrow and a charging rhinoceros experience a head-on collision. Which of the following is correct?

The sparrow experiences a smaller acceleration because it has a smaller mass. The sparrow experiences a smaller force because it has a smaller mass. The sparrow experiences a larger acceleration because it has a smaller mass. The accelerations of the sparrow and rhino are the same.

A large box is being pushed straight across the floor at a constant speed. What can you conclude about the forces acting on the box?

The amount of force applied to move the box at a constant speed must be equal to the amount of the frictional forces that resist its motion. The amount of the force applied to move the box at a constant speed must be more than its weight. The amount of force applied to move the box at a constant speed must be more than the amount of the frictional forces that resist its motion. The amount of force applied to move the box at a constant speed must be equal to its weight.

A man is standing on the ground. Which of the following pairs of forces acting on the man is an action-reaction pair as described by Newton's third law?

His weight downward, and the contact force with the ground upward. The force he exerts downward on the ground, and the contact force upward from the ground on him. The upward buoyant force, and the downward force of gravity. There are no forces on him because he is not moving.

A police officer warns you that in a traffic accident an occupant of the front seat of a car can be "thrown" through the windshield if he is not wearing a seat belt. Which of the following is a correct explanation for what the officer is describing?

The force of impact throws the occupants of the car forward. When you impact, the force is backward so there is an opposite and equal force forward on the occupants. You won't really go through the windshield; they only say that to get you to wear seatbelts. You are not thrown forward, you just keep on moving in the direction you were originally going until something stops you.

A car is going down a hill. The car is speeding up, so the driver of the car applies the brakes. He brakes just hard enough so that the frictional forces exactly equal the force that was accelerating the car down the hill. What happens?

The car will gradually slow to a stop. The car will slow down, but not stop. The car will continue on at constant speed. The car will still speed up, but not as rapidly.

A car is accelerating to the right. What direction is the frictional force on the car's tires from the ground?

Upward Downward Right Left

Newton's second law deals with the rate of change of which of the following quantities?

Position Energy Velocity Force

A tow truck is towing a small car at a steadily increasing speed. What is true about the forces between the two?

The force the car exerts on the truck is greater than the force the truck exerts on the car. The force the car exerts on the truck is less than the force the truck exerts on the car. The force the car exerts on the truck is the same as the force the truck exerts on the car.

A box is being pushed across a flat horizontal surface at constant speed. When the person applying the forward force stops pushing on it, what happens?

The box immediately stops. The box gradually stops The box continues at constant speed.

Suppose you see an object moving in a circle at constant speed. What can you say for sure about the force or forces acting on it?

The total force is toward the center of the circle. The forces are all equal since it is traveling at a constant speed. The total force pulls away from the center of the circle. The total force is zero because its speed is constant, but the acceleration is not zero because it is rotating. The total force and acceleration are both zero.

Why does a passenger in a turning car feel thrown sideways as the car turns?

The person is thrown sideways by the centripetal force. The person has a gravitational attraction to the door of the car. There is always an outward force when you travel in a circle. The person tries to keep going straight rather than around the corner, until the door pushes in on him.

A ball is tossed straight up and caught by a passenger in a car moving at a constant velocity. Which of the paths in the diagram below best represents the path of a ball as seen by someone on the ground? Four trajectories: A. a triangle-shape, B. a vertical line, C. a downward opening parabola, D. a downward opening rectangular shape.

Four trajectories: A. a triangle-shape, B. a vertical line, C. a downward opening parabola, D. a downward opening rectangular shape.

A B C D

In which of the following situations will the speed of an object increase?

The object has positive acceleration. The object has positive velocity. The object's velocity and acceleration are both negative. The object's velocity and acceleration are perpendicular.

For an object moving in a circle at constant speed the acceleration of the object is:

Perpendicular to the velocity Parallel to the velocity Zero

When you are on a ferris wheel, you feel like you weigh less at the top and like you weigh more at the bottom. Why?

As your distance from the Earth increases, your weight decreases. As you get closer to the ground, your weight increases. Your weight is pretty much the same. But, at the top, you're accelerating down and at the bottom you're accelerating up so the contact force is smaller at the top and larger at the bottom. Because you are moving in a circle there is an extra force besides gravity and the contact force. The centripetal force pulls up on you and supports your weight at the top, and pushes down on you and adds to your weight at the bottom. It's an illusion. Since you're moving at a constant speed, the forces on you must be constant.

If you apply exactly the same net force to a 5 lb and a 10 lb dumbell, what will happen?

The 10 lb dumbell will accelerate more. The 5 lb dumbell will accelerate more. The net force is the same, so they accelerate the same.

In which of the following situations will the speed of a car remain constant?

The net force on the car is equal to its velocity. The net force on the car is parallel to its speed. The net force on the car is perpendicular to its velocity. The car is moving in a circle. The forward force on the car is equal to its weight.

A car is traveling in a circle at constant speed. What can you conclude about the forces on the car.

There is a net centrifugal force pointing out from the center of the circle. The centripetal force balances the centrifugal force, so there is a net force of zero on the car. There is a net force which points in the direction the car is moving. There is a net force which points exactly toward the center of the circle. The net force has components in the direction of motion and perpendicular to the direction of motion.

A 130 lb girl stands in an elevator on a scale as the elevator just begins to move upward from the first to second floor. Which choice could properly describe her weight, contact force, and acceleration at this instant?

Weight = 130 lbs downward, contact force = 130 lbs upward, acceleration is zero. Weight = 130 lbs downward, contact force = 130 lbs upward, acceleration is downward. Weight = 130 lbs downward, contact force = 130 lbs upward, acceleration is upward. Weight = 140 lbs downward, contact force = 130 lbs upward, acceleration is downward. Weight = 130 lbs downward, contact force = 140 lbs upward, acceleration is upward. Weight = 130 lbs downward, contact force = 140 lbs upward, acceleration is downward.

Now the elevator and the girl standing on the scale are mid-floor and are moving at a constant speed upward. Which choice could properly describe her weight, contact force, and acceleration at this point?

As the elevator approaches the second floor, it begins to slow to a stop. Which choice properly describes her weight, contact force, and acceleration at this point?

Weight = 130 lbs downward, contact force = 130 lbs upward, acceleration is zero. Weight = 130 lbs downward, contact force = 130 lbs upward, acceleration is downward. Weight = 130 lbs downward, contact force = 130 lbs upward, acceleration is upward. Weight = 140 lbs downward, contact force = 130 lbs upward, acceleration is downward. Weight = 130 lbs downward, contact force = 120 lbs upward, acceleration is upward. Weight = 130 lbs downward, contact force = 120 lbs upward, acceleration is downward.

A bullet is fired horizontally from a high-powered rifle. At the same time, a second bullet is simply dropped from the same height as the bullet which was fired from the rifle. Neglecting air resistance, which statement is true about the acceleration of the first bullet after it leaves the gun compared to that of the second bullet as it falls?

The acceleration of the first bullet is much less than the acceleration of the bullet that was dropped. The acceleration of the first bullet is a little less than the acceleration of the bullet that was dropped. The acceleration of the first bullet is the same as the acceleration of the bullet that was dropped. The acceleration of the first bullet is a little more than the acceleration of the bullet that was dropped. The acceleration of the first bullet is much more than the acceleration of the bullet that was dropped.

A raindrop, as it falls to the ground, reaches a speed known as terminal velocity where the speed of the raindrop is neither increasing nor decreasing. This motion of the raindrop at terminal velocity can be understood in terms of:

One vertical force One horizontal force Two vertical forces which balance Two vertical forces which do not balance Two horizontal forces which balance Two horizontal forces which do not balance

Instructions for the next four questions

In the picture below, a boy on a sled is stopped at the top of a gentle hill. He starts down the hill, sleds at a constant speed for a short time, then slows down and stops at the bottom. In the following questions, you are asked to describe what is happening at various points along his path.

A boy sleds down a hill, and five points are depicted. 1. stopped at the top of the hill; 2. speeding up; 3. moving at a constant speed; 4. Slowing down; 5. Stopping at the bottom.

When would the boy's acceleration be directed backwards (opposite the direction he is moving)?

At none of the points below. It is forwards as long as he is moving forwards. 2 3 4 All three of the above points

When would the boy's acceleration be directed forwards (in the same direction he is moving)?

At none of the points below. It is forwards as long as he is moving forwards. 2 3 4 All three of the above points

Which of the following best explains what happens at the bottom of the hill?

The sled instantly comes to rest because there are no more forces acting on it, and therefore it stops immediately. The sled slowly comes to rest because there are no more forces acting on it, and therefore its momentum runs out and it stops gradually. The sled slowly comes to rest because of gravitational forces. The sled slowly comes to rest because of a net force acting backwards. The sled instantly comes to rest because all the forces acting on it are now balanced, so there is nothing to keep it moving.

At which of the points listed below would the net force on the sled be zero?

At none of the points below, the force can't be zero while the sled is moving. 2 3 4 at all of the points 2-4

The boy and the sled exert forces on each other. When would the contact force the sled exerts on the boy be equal to the contact force the boy exerts on the sled?

The two forces are always equal and opposite. 2 3 4 5

A cannonball is shot horizontally from a cannon. At the same time a marble is dropped from the same height as the cannon. Which object will hit the ground first? (neglect air resistance)

The cannonball Neither, they'll both hit at the same time The marble

Raindrops fall for several seconds, sometimes minutes, before reaching the ground. Why don't they strike the earth with speeds reaching hundreds of kilometers per hour?

They have a very small mass, so they experience a very small acceleration. They originate in clouds which are beyond the gravitational pull of the earth. They only speed up until the force of air resistance balances the force of gravity. Liquids do not respond to gravitational forces.

Two metal balls are the same size, but one weighs twice as much as the other. The balls are dropped from the top of a two story building at the same instant of time. The time it takes the balls to reach the ground below will be:

Half as long for the heavier ball. Half as long for the lighter ball. The same for both balls.

Three identical balls are thrown at the same time, one is thrown upward, one horizontally, the other downward. Which will hit the ground first?

The one thrown upward. The one thrown downward. The one thrown horizontally. The one thrown up hits last, the others tie for first. The one thrown horizontally hits last, the others tie for first. The one thrown down hits last, the others tie for first. They all hit at exactly the same time.

A loose wheel falls off of an airplane as it is in level horizontal flight from left to right. As seen from the ground, which path would the wheel most closely follow after leaving the plane? A depiction of four dropped trajectories. A. a parabola moving backward; B. falling straight down; C. a parabola moving forwad; D. a path moving straight forward and later straight downward.

A depiction of four dropped trajectories. A. a parabola moving backward; B. falling straight down; C. a parabola moving forwad; D. a path moving straight forward and later straight downward.

A B C D

According to the universal law of gravitation which of the following is correct?

The acceleration due to gravity is a constant everywhere. The gravitational force doesn't depend on mass. The greater the mass, the greater the acceleration. If you increase the distance between two masses, you decrease the gravitational force between them.

How does the sun's gravitational pull on the moon compare to its pull on the Earth?

The pull is about the same, because both the Earth and the moon are essentially the same distance from the sun. The sun's pull on the moon is less because the moon's mass is less. The pull on the moon is larger because the moon has less atmosphere to block the gravity. The sun's pull on the moon is zero, because the moon is in the Earth's gravitational field, not the sun's.

Suppose we could send a manned mission to a distant asteroid about a mile in diameter. When the astronauts stepped out onto the surface of the asteroid what would they discover about their weight?

They are weightless because the asteroid has no rotational motion. They are weightless because an asteroid has no atomsphere. They are almost weightless because an asteroid that size has a very small gravitational field. They weigh more because they are close to the center of the asteroid. They weigh the same because weight is independent of position.

A man jumps out of an airplane, falls for a while and then opens his parachute. After a short time, he is falling with constant speed straight down. What forces are acting on the man as he falls with constant speed?

1 force down 1 force up 2 forces which balance 2 forces which do not balance No forces

What is a centripetal force?

A force that acts in a lot of directions at once A force that is directed sideways to the direction of motion A force that causes the speed of an object to increase or decrease A force in the vertical direction

A satellite is moving around the earth with constant speed in a nearly circular orbit. What forces are acting on the satellite?

One unbalanced gravitational force A gravitational force balanced by the centrifugal force Just a centrifugal force One force keeping the satellite moving and another force keeping it in orbit

An object is initially moving under the influence of gravity. How does air friction change the acceleration of this object as the object increases in speed?

All objects accelerate the same amount regardless of speed. The acceleration of the object increases as its speed increases. The acceleration of the object decreases as its speed increases.

According to the course material, what is the suggested procedure to analyze forces on an object?

Choose an object; is gravity important? Is the object charged, carrying current, or magnetic? What does the object touch? Is friction important? Choose an object; is gravity important? How fast is the object moving? What are the 3rd law reaction forces on the nearby objects? Identify all the gravitational, electrical, and contact forces on all of the objects discussed in the problem. Choose an object; is gravity important? Is the object charged? What are the 3rd law reaction forces on the nearby objects?

You would weigh slightly less standing on the top of mount Everest than you do on the beach. Why?

You are farther from the center of the Earth. The air is thinner on top of mount Everest. Your mass is less there. You are closer to the moon, so its pull is stronger. Your are closer to space, and thing are weightless there.

Free Response

First Law

Provide an example of an object at rest that is experiencing no forces.
Provide an example of an object in uniform motion that is experiencing no forces.
Provide an example of an object at rest that is experiencing two or more forces. What forces act on the object? How do their sizes compare?
Provide an example of an object in uniform motion that is experiencing two or more forces. What forces act on the object? How do their sizes compare?
Why doesn't the First Law apply to an object experiencing a single force?
Do the First and Second Laws of Motion ever apply to the same object at the same time? Explain.

Second Law

Provide two examples of an object experiencing an unbalanced force in the direction it is moving. What do these objects have in common?
Provide two examples of an object experiencing an unbalanced force opposite the direction it is moving. What do these objects have in common?
Provide two examples of an object experiencing an unbalanced force perpendicular to the direction it is moving. What do these objects have in common?
Give two examples of an object with zero velocity that is accelerating.
Give two examples of an object accelerating while moving at constant speed.
How many forces are acting on the moon? What is causing these forces? Why does the moon move around its orbit? Why doesn't it hit Earth?
If you have 2 balls, a 1kg ball and a 2kg ball, and you push on them with the exact same force, how do the accelerations compare? Why?
Because all objects near the surface of Earth experience the same gravitational acceleration when dropped, does this mean that they are experiencing the same gravitational force? Why or why not?
On Earth, people often exercise by playing catch with a medicine ball (an extremely heavy ball). Would the same exercise work on a space station? Why or why not?
In science, the word "weight" means the gravitational force acting on an object. Using this definition, are objects in orbit around Earth weightless? Explain your answer.

Third Law

When a book is at rest on a table, its weight and the contact force from the table are equal and opposite. Are these forces an action-reaction pair? Explain your answer.
10-year-old Brianna challenges her 4-year-old sister Lindsey to an arm wrestling match. Brianna wins without trouble. How is this possible if Lindsay pushes on Brianna as hard as Brianna pushes on Lindsey?
The BYU Society of Physics Students (SPS) challenges the BYU football team to a tug of war, with the one condition: that the SPS get to pick the playing field. The football team accepts, and the SPS announce that the match will take place at the Seven Peaks Ice Arena with the football team on the ice and the physics students standing on the concrete in the entrance to the rink. The SPS will try to pull the football players off the ice onto the concrete, and the football players try to pull the SPS off the concrete onto the ice. The first team to have a player touch the boundary between ice and concrete wins. Use Newton's Second and Third Laws to explain the strategy of the SPS.