AP Physics 1 : Specific Forces

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

Example Question #31 : Forces

What is the mass of  bird?

Possible Answers:

Correct answer:

Explanation:

Weight is the force that gravity exerts on an object. Weight is determined using the equation

Where  is the mass of the object and  is the gravitational constant on the given planet.

Given the weight of an object and the gravitational constant acting on it, we find the mass using

Example Question #591 : Newtonian Mechanics

If a person has a mass of  on Earth, what is the mass and weight of a person on Jupiter, where ?

Possible Answers:

Correct answer:

Explanation:

Weight is the force that gravity exerts on an object. Weight is determined using the equation

Where  is the mass of the object and  is the gravitational constant on the given planet.

The mass of an object is independent of gravity, therefore it does not change from planet to planet. The mass of the person on Jupiter is .

On Jupiter, , so the weight of the person on Jupiter is:

Example Question #33 : Specific Forces

A shield has a mass of  on Earth, what is the mass and weight of the shield on the moon, where ?

Possible Answers:

Correct answer:

Explanation:

Weight is the force that gravity exerts on an object. Weight is determined using the equation 

Where  is the mass of the object and  is the gravitational constant on the given planet.

The mass of an object is independent of gravity, therefore it does not change from planet to planet. The mass of the shield on the moon is .

On the moon, , so the weight of the shield on the moon is:

Example Question #31 : Normal Force And Weight

Suppose that a person standing in an elevator suddenly accelerates upward to go to the next floor. If the elevator accelerates upward by an amount , by what factor will this person's weight change?

Possible Answers:

There person's weight will remain the same

There person's weight will decrease by a factor of 

There person's weight will increase by a factor of 

The person's weight will decrease by a factor of 

The person's weight will increase by a factor of 

Correct answer:

The person's weight will increase by a factor of 

Explanation:

For this question, we're told that a person is accelerating upward while in an elevator, and we're asked to find how this person's weight changes as a result.

To begin, it's most useful to solve this problem by considering the forces acting in the vertical direction. Downward, we have the mass of the person. Upward, we have the tension in the cord that is pulling the elevator up. We can use this information to set up the following expression.

Also, note that the tension in the elevator is what is providing all the upward force. Thus, the elevator floor will be exerting this same amount of force on the person (normal force) and, as a result, the person will exert the same force (weight) on the elevator floor. Hence, tension is a measurement of the person's weight.

Now, we'll need to take the ratio of the person's weight while accelerating in the elevator to the person's weight while at rest. This will give us the factor by which his weight changes.

Example Question #31 : Normal Force And Weight

A girl is standing on two feet on the ground. She then lifts her right foot in the air. How does this change the normal force affecting her?

Possible Answers:

No change

None of these

It will be quadrupled

It will be cut in half

It will be doubled

Correct answer:

No change

Explanation:

Normal force is independent of surface area, and only dependent on the opposite force being applied, in this case gravity. While the pressure on her foot may change, the force of gravity pushing her down stays the same.

Example Question #602 : Newtonian Mechanics

A  person stands on solid ground without moving. How much force is exerted on the person by the ground? Assume gravity is .

Possible Answers:

Correct answer:

Explanation:

The ground exerts an equal force as the person exerts on it, which is the normal force. This is simply the mass of the person times the acceleration due to gravity which is . This ends with the normal force and the force exerted by the ground on the person being .

Example Question #32 : Forces

A  block remains stationary on the ground. Assuming that the acceleration due to gravity, , is , and that the only forces acting on the block are the force of gravity and normal force, what is the value of the normal force exerted by the ground on the block?

Possible Answers:

Correct answer:

Explanation:

This question tests your understanding of the concept of normal force. In a block that remains stationary, in which the only two forces acting upon it are the force of gravity, and normal force, you therefore know that the normal force must be equal to the force of gravity, since the block is not accelerating or decelerating (i.e. the sum of the forces is equal to zero). 

Thus, we can set the normal force equal to the force of gravity, and solve as follows:

Therefore, the normal force exerted on the block by the ground is .

Example Question #1 : Universal Gravitation

A certain planet has three times the radius of Earth and nine times the mass. How does the acceleration of gravity at the surface of this planet (ag) compare to the acceleration at the surface of Earth (g)?

Possible Answers:

Correct answer:

Explanation:

The acceleration of gravity is given by the equation a_{g} = \frac{GM}{r^{2}}, where G is constant.

For Earth, a_{g} = \frac{GM_{earth}}{r_{earth}^{2}} = g.

For the new planet, 

.

So, the acceleration is the same in both cases.

 

Example Question #1 : Universal Gravitation

A new planet is discovered with mass  and with a diameter of . What is the lowest escape velocity required to escape this planet's gravitational pull?

Possible Answers:

Correct answer:

Explanation:

The equation to calculate the escape velocity from a planet is

 

The diameter of the planet is given and can be divided by two to determine the radius of the planet. By plugging in the given values, the escape velocity threshold can be determined:

Example Question #1 : Universal Gravitation

There are two isolated stars orbiting each other. The first star has a mass of  and the second star has a mass of . If the stars are 2,000km away, what is the gravitational force felt by the first star?

 

Possible Answers:

None of the other answers

Correct answer:

Explanation:

We need to know Newton's law of universal gravitation to solve this problem:

It is important to note that both suns will feel the same gravitational force. However, since they have different masses, they will accelerate at different rates.

Plugging in the variables we have, we get:

 

 

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