Forces and Free-Body Diagrams AP Style Practice

A bucket rests on top of a crate, which rests on a floor, as shown in the diagram. The bucket is lifted off the crate. How does the force due to microscopic interactions between the crate and the floor change, if at all, as the bucket is lifted off the crate?

• It decreases.

• It increases.

• It stays the same.

• It could either increase or decrease depending on the materials that the crate and floor are made of.

Answer explanation

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NGSS.HS-PS2-1

A cart rolls along a track that is inclined at an angle of 37° to the horizontal, as shown. At the moment shown, there are three forces exerted on the cart: a downward force of magnitude 50 N, a drag force of magnitude 20 N, and a normal force from the track of magnitude 40 N, as shown in the free-body diagram.

The magnitude of the vector sum of the forces on the cart is most nearly

• 10 N

• 20 N

• 0 N

• 110 N

Answer explanation

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What is the magnitude of the vector sum of the forces exerted on the puck?

• 3 N

• 4 N

• 5 N

• 7 N

Answer explanation

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The forces exerted on an object at a particular instant are represented in the free-body diagram. The magnitude of each force is drawn to scale. A student claims that the vector sum of the forces on the object is equal to zero. Is the student’s claim valid?

• Yes, because the horizontal components add to zero.

• Yes, because the vertical components add to zero.

• No, because the vertical components of the forces do not completely add to zero.

• No, because the magnitudes of all the forces are not equal

Answer explanation

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In which of the following free-body diagrams is the magnitude of the vector sum of the forces the greatest?

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Answer explanation

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The free-body diagram shows the three forces exerted on an object. Each square in the grid is 1 N by 1 N. What is the magnitude of the vector sum of the forces exerted on the object?

• 0 N

• 2 N

• 6 N

• 7.7 N

Answer explanation

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The left figure shows a block at rest on a rough inclined surface. The block is attached to a wall by a string of negligible mass. The vector sum of the forces is zero. The right figure shows the free-body diagram for the block. The string is cut and the vector sum of the forces on the block remains zero. Which of the following free-body diagrams could show the forces exerted on the block after the string is cut?

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Answer explanation

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A hollow plastic ball hangs at rest from a string at an angle, as shown. There is a force on the ball from the string directed up and to the left, and a downward gravitational force. There is also a constant flow of air around the ball, and the vector sum of all the forces exerted on the ball is equal to zero. Which of the following statements correctly indicates a possible direction of the force exerted on the ball by the air, and provides a valid justification?

• To the right, because it will balance the leftward component of the force from the string

• To the right, because it will balance the downward gravitational force

• Down and to the left, because it will balance the force from the string

• Down and to the left, because it will balance the downward gravitational force

Answer explanation

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A platform of mass m is held at rest by two ropes connected to its ends, as shown in the diagram on the left. A block, also of mass m, is placed on the center of the platform. A student draws a free-body diagram representing the forces exerted on the platform while it is at rest, as shown in the diagram on the right. Which of the following claims best explains why the diagram is incorrect?

• The tension forces from the ropes do not point in the correct directions.

• The diagram does not include components of the tension forces in the ropes.

• The horizontal forces are unbalanced.

• The vertical forces are unbalanced.

Answer explanation

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NGSS.HS-PS2-1

1. A ball is thrown and follows a parabolic path, as shown. Air friction is negligible. Point Q is the highest point on the path.  Which of the following best indicates the direction of the net force on the ball at point P?

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Answer explanation

The only force currently acting on the projectile is the force of gravity.

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