AP Physics 1 : AP Physics 1

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

Example Question #11 : 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:

Explanation:

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 #31 : Fundamentals Of Force And Newton's Laws

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:

Explanation:

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 #12 : 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:

Explanation:

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 #13 : 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:

Explanation:

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 #14 : Newton's Second Law

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

Possible Answers:

Correct answer:

Explanation:

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 #14 : 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:

Explanation:

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

Example Question #21 : Newton's Second Law

Suppose that an astronaut on the moon applies a horizontal force of  to a  mass that is initially at rest. Considering that the acceleration due to gravity on the moon is  and that there is no friction, what is the acceleration of this object?

Possible Answers:

Correct answer:

Explanation:

In this question, we're told that a horizontal force is being applied to an object on the moon. We're given the magnitude of this force and the mass of the object.

We're also given the acceleration due to gravity on the moon, but this is useless information for the purposes of this question. Since the force under consideration is acting horizontally on the object, and the force due to gravity acts vertically, these two forces are treated separately; the vertical acting force of gravity will not affect the horizontal kinematics of the object's movement.

Thus, this reduces to a simple application of Newton's second law, and we can use the following equation:

Rearranging, we can isolate the term for acceleration:

Then plug in the values that we have to solve for our answer:

Example Question #22 : Newton's Second Law

Consider the following system:

 

Spinning rod with masses at end

Two spherical masses, A and B, are attached to the end of a rigid rod with length l. The rod is attached to a fixed point, p, which is at a height, h, above the ground. The rod spins around the fixed point in a vertical circle that is traced in grey.  is the angle at which the rod makes with the horizontal at any given time ( in the figure).

The rod is initially at rest and held in the horizontal position. When it is released, what is the initial instantaneous acceleration of mass A, and in what direction? Assume the rod has a negligible mass. Neglect air resistance and internal frictional forces.

Possible Answers:

None of the other answers

Correct answer:

Explanation:

This problem can be solved using the expression relating torque and angular acceleration:

We can determine the total net torque on the system using the Newton's second law.

Applying this to both masses and assuming a force in the downward direction is positive, we get:

Then using the expression for torque, we get:

Where d is half the length of the rod.

Applying this to both masses and assuming torque in the counterclockwise direction is positive, we get:

Now let's go back to the original equation:

Now we need to calculate the net moment of inertia:

 

Where r is half the length of the rod:

Going back to the original equation and rearranging for angular acceleration:

Then relating this to linear acceleration using:

Where r is half the length of the rod again:

We have all of our values, so we can calculate the final answer:

Example Question #21 : Newton's Second Law

Consider the following system:

Frictionless pully

Two masses, A and B, are attached to the end of a rope that runs through a frictionless pulley.

 

The two masses are initially held at rest. When they are released, what is the initial instantaneous acceleration of mass A, and in what direction? Neglect air resistance and any frictional forces.

 

Possible Answers:

Correct answer:

Explanation:

We can use Newton's second law:

There are two forces acting on the system: gravity on each mass. For simplicity sake, we will examine these forces with respect to mass. Therefore, the force on mass B becomes an upward tension force on mass A. Now we need to clarify which direction is positive. For this problem, we'll say that a downward force is positive, and an upward force is negative.

We'll start with the gravitational force:

Now for tension. The tension force is simply the gravitational force applied to mass B:

Note that it's negative because it is in the upward direction

Now adding these together to get the net force:

Substituting this back into the original equation, we get:

Where m is the combination of both masses:

Rearrange for acceleration:

We know each value, so we can solve the problem

Example Question #21 : Newton's Second Law

Consider the following system:

Frictionless pully

Two masses, A and B, are attached to the end of a rope that runs through a frictionless pulley.

 

The two masses are initially at rest. The pulley has a radius . The masses are released. After , what is the rotational velocity of the pulley? Neglect air resistance and any frictional forces.

Possible Answers:

Correct answer:

Explanation:

In order to determine rotational velocity of the pulley, we need the linear velocity of the outside of the pulley, which is simply the linear velocity of the rope/masses. We can determine that using the acceleration of the masses and the length of time given. Therefore, we will begin with Newton's 2nd law applied to one of the two masses. We will arbitrarily apply it to mass A:

There are two forces applied to mass A, gravity and tension from the rope. We will assume that a downward force is positive, and an upward force is negative. First, let's start with gravity:


Next we have tension, which is just the gravitational force applied on mass B:

Adding these together to get net force:

Substituting this into the original equation:

Rearranging for acceleration:

Now that we have acceleration, we can use it to determine linear velocity after the stated length of time:

Now we can use this to determine rotational velocity:

We know all of these values, so time to plug and chug:

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