Physics Forces Quiz 06

Which statement explains why a book resting on a table is in equilibrium?

• There is a net force acting downward on the book.

• The weight of the book equals the weight of the table.

• The acceleration due to gravity is 9.8 m/s/s for both the book and the table.

• The weight of the book and the table’s upward force on the book are equal in magnitude, but opposite in direction.

Answer explanation

The book is in equilibrium because the downward force of its weight is balanced by the upward force exerted by the table. These forces are equal in magnitude and opposite in direction, resulting in no net force.

A book that is resting on a shelf 1.5 m above the floor has a weight of 12 N. The force the shelf exerts upon the book must be:

• 8 N

• 12 N

• 18 N

• 24 N

Answer explanation

The force the shelf exerts on the book is equal to the weight of the book due to Newton's third law. Since the book weighs 12 N, the shelf must exert a force of 12 N upward to support it.

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While closing a door of mass 10 kg, a student pushes on it with a force of 25 N. As the door is accelerating to close, what force does the door exert on the student?

• Depends on the acceleration of the door.

• 5 N

• 25 N

• 35 N

• 250 N

Answer explanation

According to Newton's third law, the force exerted by the door on the student is equal in magnitude and opposite in direction to the force the student applies. Therefore, the door exerts a force of 25 N on the student.

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NGSS.MS-PS2-1

A man is attempting to push a chair across the floor with constant speed by exerting a force of 50 N. When will the chair exert a force of 50 N on the man?

• Only when the man and the chair are at rest.

• Only when the man and the chair are accelerating.

• Only when they are at rest or moving with constant velocity.

• Never. The man’s push is always greater than the chair’s.

• When the chair is at rest or in motion.

Answer explanation

According to Newton's third law, the chair exerts an equal and opposite force on the man. Therefore, the chair will exert a force of 50 N on the man whether it is at rest or in motion, making the correct choice: 'When the chair is at rest or in motion.'

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A man standing on a chair weighs 800 N. In order for the chair to support the man, the chair must exert an upward force of:

• more than 800 N.

• less than 800 N.

• 800 N.

• It depends on how many pounds there are in 800 N.

Answer explanation

The chair must exert an upward force equal to the man's weight to support him. Since the man weighs 800 N, the chair must exert an upward force of 800 N to maintain equilibrium.

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A student is holding a book that has a weight of 20 N in his hand while sitting in a chair. A man claims that the book must be attracting Earth with a force of 20 N. His claim must be:

• false because Earth is much larger than the book.

• true because of force pair interactions.

• false because books do not attract objects.

• true because the book has more inertia than Earth.

Answer explanation

The man's claim is true because of Newton's third law: for every action, there is an equal and opposite reaction. The book exerts a gravitational force of 20 N on Earth, and Earth exerts the same force back on the book.

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Which diagram of a 5-kg mass resting on a table correctly represents the force of the table on the mass?

• a)

• b)

• c)

• d)

Answer explanation

The correct choice, a), shows the force of the table acting upward on the mass, balancing the downward gravitational force. This represents Newton's third law, where the table exerts an equal and opposite force on the mass.

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NGSS.MS-PS2-2

A ball that is made of rubber bounced back into the air after it hit the ground. While the ball is in the air and moving up, is it exerting any forces?

• No, the ball is not touching anything, so it is not exerting any forces.

• Yes, the ball is pulling on the earth with a gravitational force.

• No, but the air exerts a force on the ball.

• Yes, the ball is still pushing on the earth from the bounce since the ball is still going up.

Answer explanation

Yes, the ball is pulling on the Earth with a gravitational force. Even while in the air, the ball exerts a force due to gravity, which is a fundamental interaction between the ball and the Earth.

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A ball is rolled along and off a table. Which of the following acts on the ball after it leaves the table but before it hits the floor?

• Only the force of the earth’s gravity on the ball.

• Only the force the ball got from being rolled off the table.

• Both the force of the earth’s gravity on the ball and the force the ball got from being rolled off the table.

• Neither the force of the earth’s gravity on the ball nor the force the ball got from being rolled off the table.

Answer explanation

After the ball leaves the table, the only force acting on it is the force of the earth's gravity. The initial force from being rolled does not act on the ball anymore; it is now in free fall.

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NGSS.MS-PS2-2

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A spring scale reads 20 Newtons as it pulls a 5.0-kilogram mass across a table. What is the magnitude of the force exerted by the mass on the spring scale?

• 0.25 N

• 4 N

• 5 N

• 20 N

• 100 N

Answer explanation

The spring scale measures the force exerted on it. Since it reads 20 Newtons, this is the force the mass exerts on the scale, according to Newton's third law. Therefore, the correct answer is 20 N.