### All AP Physics 1 Resources

## Example Questions

### Example Question #11 : Newton's Second Law

In which of the following situations can we claim the net force is zero on the object described?

**Possible Answers:**

A child swinging on a swing set.

A bird taking flight from the ground.

Airplane changing direction to avoid a storm while maintaining a constant speed.

Elevator ascending at a constant speed.

Car breaking to a stop.

**Correct answer:**

Elevator ascending at a constant speed.

Acceleration requires a change in one of two quantities: speed and direction

The elevator is traveling only upwards and at a non changing speed. The accleration is zero and thus the net force must be zero by newton's second law.

The bird taking flight is accelerating from rest to some non-zero speed. Net force is non zero because the speed changes.

The child on the swing set changes direction due to traveling in a semi-circle. Also their speed will change as well. Net force is non zero because both the speed and direction change.

The car breaking to a stop is changing speed. Net force is non zero because the speed changes.

The airplane, while maintaining a constant speed, changes direction and therefore accelerates. Net force is non zero because the direction changes.

### Example Question #11 : Newton's Second Law

A proton has a mass that's about times the mass of an electron. Given that electrostatic forces between a proton and electron cause them to attract one another with the same force, what can you say about the acceleration of the two particles?

**Possible Answers:**

The acceleration of the electron is of the acceleration of the proton.

The acceleration of the proton is the same as the acceleration of the electron.

The acceleration of the proton is of the acceleration of the electron

The acceleration of the proton is times the acceleration of the electron.

**Correct answer:**

The acceleration of the proton is of the acceleration of the electron

For this, we have to know that:

, where is a force, is an object's mass, and is its acceleration.

The text tells us that the electrostatic force between the proton and electron will exert the same force, but that the mass of the proton is times the mass of the electron.

, where is the force on the proton and is the force on the electron.

To determine the effect on their acceleration:

, where and are the masses of the proton and electron respectively, and and are the acceleration of the proton and electron respectively.

Since we know that

Solving for the acceleration of the proton:

The acceleration of the proton is of the acceleration of the electron.

### Example Question #12 : Newton's Second Law

If a object is subjected to a force of , by how much will it accelerate?

**Possible Answers:**

**Correct answer:**

In this question, we're being told that an object of a given mass is being subjected to a force. To solve this problem, we'll need to make use of Newton's second law, which states that an object of a given mass will accelerate at a rate that is proportional to the force that is applied. Or, written in equation form:

Plugging in the values given, we obtain:

### Example Question #13 : Newton's Second Law

2 objects(named object A and object B) of equal masses and initial kinetic energy collide onto one another. During the collision, object A loses of its kinetic energy, which object B gains. Assume mass of both objects remain unchanged.

Given that object A's mass is and its velocity changes by over a period of seconds, determine the average force applied on object A.

**Possible Answers:**

**Correct answer:**

Force is given by:

, where is mass and is acceleration.

We're given mass, but we aren't given acceleration. Since the question asks for average force , we can determine average acceleration .

, where is the change in velocity and is change in time.

In our problem, and

### Example Question #14 : Newton's Second Law

A block with a mass of is pushed across a frictionless surface with a force of for a time of . What is the velocity of the block after the push?

**Possible Answers:**

**Correct answer:**

Here we must use the following formula:

We can substitute our known values of mass and force and solve for acceleration

Since we know the acceleration and the time it acts upon the object, we can determine the final velocity through the following equation:

### Example Question #15 : Newton's Second Law

Consider a block sitting at rest on an inclined plane. Find the maximum inclination angle the plane may have without the block sliding if the coefficients of kinetic and static friction are , respectively.

**Possible Answers:**

**Correct answer:**

If the block is to remain at rest on the plane, we know that the sum of the forces acting a long the plane must be equal and opposite. This means that the gravitational force acting along the plane is equal to and opposite of the force of friction. This can be demonstrated as:

This can be rewritten as:

For the block to remain at rest, the force of static friction must exceed (for this problem we will set them equal to each other since it gives us the best approximation); solve for the angle:

### Example Question #16 : Newton's Second Law

A force of is applied to a object in space.

What is the acceleration of the object?

**Possible Answers:**

**Correct answer:**

Newton's second law states:

Where is the net force exerted upon an object, is the mass of the object and is the acceleration of the object.

We rearrange this equation to show:

Plug in our given values with and :

### Example Question #11 : Newton's Second Law

If of force is continuously applied to a box with mass , what will the box's velocity be after given that it's initial velocity was ?

**Possible Answers:**

**Correct answer:**

By Newton's second law:

, where is force, is mass, and is acceleration.

This is the acceleration. Since we're assuming this acceleration is constant over time, we can model velocity as:

where is the initial velocity.

Since the initial velocity is in our problem,

After seconds,

### Example Question #18 : Newton's Second Law

A train of mass goes from to in . Calculate the deceleration in terms of .

**Possible Answers:**

**Correct answer:**

Use work:

All energy will be kinetic.

Convert to :

Plug in values. Force will be negative as it is pointing against the direction of travel:

Solve for :

Use Newton's second law:

Plug in values:

Solve for :

Convert to

### Example Question #19 : Newton's Second Law

A train of mass goes from to in . Estimate the coefficient of friction of the steel wheels on the steel rails. Assume the wheels are locked up.

**Possible Answers:**

**Correct answer:**

Use work:

All energy will be kinetic.

Convert to :

Plug in values. Force will be negative as it is pointing against the direction of travel:

Solve for :

Use frictional force:

Plug in values:

Solve for

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