AP Physics C: Mechanics : Motion

Study concepts, example questions & explanations for AP Physics C: Mechanics

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Example Questions

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Example Question #1 : Mechanics Exam

Which of the following is not a vector quantity?

Possible Answers:

Velocity

Speed

Acceleration

Force

Correct answer:

Speed

Explanation:

Speed is defined as the magnitude of velocity. In other words, speed visually represents the size of a velocity vector, but is indepedent of its direction.

Velocity, acceleration, and force are all vector quantities. Each is defined by the magnitude of the measurement as well as its direction of action.

Example Question #1 : Ap Physics C

A dog is initially standing near a fire hydrant. The dog moves 3 meters to the right. Then it runs 7 meters to the left. What is the dog's final displacement from its original position?

Possible Answers:

Correct answer:

Explanation:

This question tests your understanding of displacement as a vector quantity. The best way to solve it is to create a number line to track the motion of the dog.

Let the initial position of the dog be 0m on our number line:

Dog1

Then the dog moves three meters to the right, which we can represent as follows:

Dog2

Then the dog moves seven meters to the left. Note that it is moving seven meters to the left from the position it is at now (3m):

Dog3

We find that at the end of its motion, the dog's displacement is four meters to the left of its original position, which we write as .

Remember that displacement is the change in position, and not the total distance traveled. We only care about the initial position and the final position, regardless of the path traveled. Positive and negative signs indicate the direction of motion.

Example Question #1 : Motion

You drive your car from your house all the way to your school which is 50km away. After you are done with classes you drive back through the same route and park exactly where you had your car at the beginning of the day. By the end of the day, what were the distance and displacement of your motion?

Possible Answers:

Correct answer:

Explanation:

This question tests your conceptual understanding of distance as a scalar quantity vs your understanding of displacement as a vector quantity. 

Distance measures the total length that was traveled in a given motion, and does not care about the direction since it is a scalar value. In your day, you traveled 50km on your way to school and 50km on your way back home. In total you traveled 100km, so that is your distance.

Displacement is a vector quantity that measures the change in position. It cares about your final and initial positions, taking into account the direction of the change in position. In this scenario you started and ended your motion exactly at the same position, so overall at the end of the day your car did not change position at all. Therefore your displacement was 0m.

Example Question #1 : Mechanics Exam

An object travels North along a straight line at a constant rate of . What are the object's speed and velocity?

Possible Answers:

Correct answer:

Explanation:

This is a simple question that tests your conceptual understanding of speed as a scalar quantity and velocity as a vector quantity. The motion of the object is quite simple, so you need only to be mindful of the fact that velocity, as a vector, must tell you both the magnitude and direction (how fast it is going and where), while speed only tells you the magnitude (how fast it is going).

Therefore, your speed is  and your velocity is .

Example Question #5 : Linear Motion

An object starts from rest and reaches a velocity of  after accelerating at a constant rate for four seconds. What is the distance traveled in this time? 

Possible Answers:

Correct answer:

Explanation:

Since the acceleration is constant in this problem, we can apply the kinematics given equation to calculate the distance:

First, we need to calculate the acceleration.

Plug in our velocity and time values to find the acceleration.

Now we can return to the kinematics equation and solve for the distance traveled:

Example Question #1 : Motion

At the moment a car is passed by another car constantly traveling at , it begins to accelerate at . In how many seconds does this car catch up to and pass the other car?

Possible Answers:

Correct answer:

Explanation:

First, find the displacement equations for both cars. Car 1 will be the car that is initially stationary; car 2 will be the car traveling with constant velocity.

Car 1:

Car 2:

Now, set those displacement equations equal to each other and solve for .

The accelerating car will catch and pass the car traveling at constant velocity after 4 seconds.

Example Question #1 : Linear Motion

An object moving along a line has a displacement equation of , where is in seconds. For what value of is the object stationary?

Possible Answers:

Correct answer:

Explanation:

Use the fact that to find the velocity equation, and then solve for when .

Take the derivative of the displacement equation.

Set the velocity equal to zero.

Solve for the time.

Example Question #1 : Ap Physics C

An object is moving on a line and has displacement equation of , where is in seconds. At what value of is the object not accelerating?

Possible Answers:

Correct answer:

Explanation:

Take the second derivative of , to find the acceleration function . Then find the value of where . The fact that it is simply  and not or inside that quantity means that using the Chain Rule becomes decidedly easier.

Set the acceleration fuction equal to zero and solve for the time.

 

Example Question #1 : Mechanics Exam

A projectile is launched out of a cannon or launch tube with zero air resistance or friction. At what angle (in degrees) should the projectile be launched to maximize the distance it travels?

Possible Answers:

Correct answer:

Explanation:

The projectile travels the farthest when the vertical component of its velocity matches the sum of its horizontal component and whatever the wind/friction adds or subtracts. If the wind has no effect, then a 45-degree angle will be the best because the horizontal and vertical components of the velocity will create a right isosceles triangle (remember special triangles).

Example Question #1 : Ap Physics C

The velocity (in meters per second) of a moving particle is given by the following function:

If the particle's initial position is 0m, what is the position of the particle after two seconds?

Possible Answers:

Correct answer:

Explanation:

To solve this problem you need to obtain a function of position with respect to time. For that, you need to understand velocity as the rate of change of displacement with respect to time. In other words, velocity is "how fast" (i.e. how much time it takes) an object changes position (remember displacement is the change in position). This means that velocity is the derivative of displacement with respect to time.

Therefore, to obtain a function of position with respect to time you need to take the antiderivative of the velocity function, so we integrate:

Here,  is a constant that represents the initial position of the particle. We know that the initial position of the particle is 0m, so our function is:

Therefore, after two seconds have passed, we have t = 2s and

Note: we know that position is given in meters since the question specified that velocity is measured in meters per second. 

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