Physics Forces Quiz 07

Tim notices that when he brings the magnet near the refrigerator, he can let go of the magnet before it touches the refrigerator and the magnet will leap to the refrigerator and stick rather than falling to the floor. Is the refrigerator exerting a force on the magnet?

• No, the energy of the magnet makes it attracted to the refrigerator.

• Yes, but only after the magnet is touching the refrigerator.

• No, the magnets can exert forces over a distance, but non-magnets, like a refrigerator cannot.

• Yes, the refrigerator is pulling on the magnet with a magnetic force.

Answer explanation

Yes, the refrigerator is exerting a magnetic force on the magnet, causing it to leap towards the refrigerator even before contact. This attraction is due to the magnetic properties of the refrigerator.

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Jennifer and Katie stand and lean on each other. Jennifer weighs 150 pounds and Katie weighs 120 pounds. Which one pushes harder on the other?

• Jennifer pushes harder because she weighs more.

• Jennifer and Katie push on each other with the same size force.

• Katie must push harder because she weighs less and has to compensate for having less weight.

• It depends on whether Jennifer or Katie moves.

Answer explanation

According to Newton's third law, Jennifer and Katie push on each other with equal and opposite forces, regardless of their weights. Therefore, they exert the same size force on each other.

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Jared is trying to decide if he will be able to push his car home after it runs out of gas. Which of the following conclusions is most likely to be true?

• Only if Jared is strong enough will he be able to push harder on the car than the car can push on him.

• The car will NOT move because it is heavier than Jared so the car pushes back on him harder than he can push on the car.

• The car may move. The motion of the car depends only on the forces acting on the car, not the force of the car pushing back on Jared.

• If the car does move it is because Jared pushed harder on the car than the car pushed on him.

• The car will NOT move because the forces on the car are equal and opposite, so there is no net force.

Answer explanation

The correct choice states that the car may move, as its motion depends on the forces acting on it. Jared's ability to push does not negate the forces on the car, making this conclusion the most accurate.

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Bob weighs 150 pounds (667 newtons). Which of the following statements best explains the gravitational force affecting Bob?

• The gravitational force acting on Bob must be more than the gravitational force he exerts on the Earth since Bob is the one who accelerates as he falls.

• Since Bob will fall "toward the Earth" if he steps off a chair, the Earth's gravity must pull on him more than he pulls on the Earth.

• The Earth pulls on Bob with 150 pounds of gravitational force, but Bob pulls with less gravitational force on the Earth since he has less mass.

• The Earth pulls on Bob with 150 pounds of gravitational force, and Bob pulls on the Earth with equal gravitational force.

Answer explanation

According to Newton's third law, the gravitational force the Earth exerts on Bob (150 pounds) is equal to the force Bob exerts on the Earth. Thus, both forces are equal, making the correct statement the one that reflects this equality.

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A communications satellite orbits the Earth. Which of the following statements best describes force interactions between the Earth and the satellite?

• The satellite is weightless in outer space.

• The satellite only feels the force of inertia.

• The Earth pulls on the satellite and the satellite pulls on the Earth equally.

• The Earth alone is pulling on the satellite, but the satellite is too far away for it to matter.

Answer explanation

The correct choice states that the Earth pulls on the satellite and the satellite pulls on the Earth equally, which aligns with Newton's third law of motion. Both forces are equal in magnitude and opposite in direction.

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In the diagram, a second block is stacked on top of the first block. How many forces are acting on the lower block?

• None

• One, gravity

• Two, gravity and the table

• Three, gravity, the table and the upper block

Answer explanation

The lower block experiences three forces: the gravitational force pulling it down, the normal force from the table pushing it up, and the weight of the upper block exerting a downward force on it. Thus, the correct answer is three.

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A box slows down as it moves to the right along a horizontal surface as shown in the diagram. Which vector best represents the direction of the force of friction on the box?

• a)

• b)

• c)

• d)

Answer explanation

The box is slowing down while moving to the right, indicating that friction opposes its motion. Therefore, the force of friction acts to the left, which is best represented by vector d).

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Sand is often placed on an icy road because the sand:

• increases the normal force of the car on the road.

• decreases the normal force of the car on the road.

• increases the coefficient of friction between the car and the road.

• decreases the coefficient of friction between the car and the road.

Answer explanation

Sand increases the coefficient of friction between the car and the icy road, providing better traction and reducing the likelihood of slipping. This helps vehicles maintain control on slippery surfaces.

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Automobiles with front-wheel drive that have the engine located over the drive wheels have better traction in snow than automobiles with rear-wheel drive. This is most likely because:

• the front tires have a higher normal force than the rear-wheel tires because the engine is heavier than the rear of the automobile.

• the front wheels are used for steering.

• the tires on front-wheel drive automobiles have a higher coefficient of friction than rear wheel drive automobiles.

• the front tires encounter the snow first.

Answer explanation

Front-wheel drive cars have the engine weight over the front wheels, increasing the normal force on them. This results in better traction in snow compared to rear-wheel drive, where the rear wheels have less weight and traction.

Suppose you are pushing a textbook across a table at constant speed. Then you push the book so that it moves at a faster constant speed. How would the force you used on the book with the fast constant speed compare to the force you used with the slow constant speed?

• You must be pushing harder to make the book move at a faster constant speed.

• The box now has a greater force of motion.

• There is a greater difference between the downward force and the upward force.

• The force you exert is still equal in size and opposite in direction to friction.

Answer explanation

When pushing the book at constant speeds, the force you exert equals the friction force opposing it. Thus, even at a faster speed, the force remains equal in size and opposite to friction, ensuring constant motion.

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