Question

As defined in Chapter 7, the work done by a force on an object is equal to the force times the displacement times the cosine of the angle between the force and displacement vectors (W=F·d cos(θ)).
Suppose you are supporting a 1.56-kg block. What is the gravitational force (magnitude and direction) acting on the block?

Up

Down

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If you lower the block a distance of 0.208 m, what is the work done by the gravitational force as you lower the block?

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What is the value of the angle θ (in degrees – do not enter units)?

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What is the change in the gravitational potential energy as you lower the block 0.208 m? (If the potential energy increases, the answer should be positive. If the potential energy decreases, the answer should be negative.)

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Let’s switch gears and ask about the electrical force on a charged object. Suppose you have a region where the electric field has a magnitude of 23.1 N/C, the field points straight down, and the field is uniform (ie., the magnitude and direction are the same everywhere in this region). You place an object which has a charge of +0.251 C in this field. What is the magnitude and direction of the electric force acting on the object?

Up

Down

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Suppose you lower the object a distance 0.208 m (in the same direction the field is pointing). What is the work done by the electrical force as you lower the block?

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What is the change in electric potential energy as you lower the object 0.208 m? (If the electric potential energy increases, the answer should be positive. If the electric potential energy decreases, the answer should be negative.)

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This discussion is closed.

Answer 1

1) Suppose you are supporting a 1.56-kg block. What is the gravitational force (magnitude and direction) acting on the block?

The magnitude of the gravitational force is given by the eq.

or

The direction of the gravitational force is always down.

2) If you lower the block a distance of 0.208 m, what is the work done by the gravitational force as you lower the block?

The work done by the gravitational force is given by the eq.

or

Notice that the angle used for function cosine is 0°, this is because both vectors are pointing downward (the gravitational force and the displacement vector).

3) What is the value of the angle θ (in degrees – do not enter units)?

Since both vectors point in same direction, the angle is 0.

4) What is the change in the gravitational potential energy as you lower the block 0.208 m? (If the potential energy increases, the answer should be positive. If the potential energy decreases, the answer should be negative.)

The change in the gravitational potential energy as you lower the block 0.208m is given by the eq.

5) Let’s switch gears and ask about the electrical force on a charged object. Suppose you have a region where the electric field has a magnitude of 23.1 N/C, the field points straight down, and the field is uniform (ie., the magnitude and direction are the same everywhere in this region). You place an object which has a charge of +0.251 C in this field. What is the magnitude and direction of the electric force acting on the object?

The electric force acting on the object is given by the eq.

since the charge is positive, the direction of the electric force is down as the electric field. The magnitude is

or

6) Suppose you lower the object a distance 0.208 m (in the same direction the field is pointing). What is the work done by the electrical force as you lower the block?

Since the displacement vector is poiting in the same direction that the force, the angle between these vectors is 0°. Thus, the work done by the electrical force is

or

7) What is the change in electric potential energy as you lower the object 0.208 m? (If the electric potential energy increases, the answer should be positive. If the electric potential energy decreases, the answer should be negative.)

The change in electric potential energy as you lower the object 0.208m is given by the eq.