AP Physics 1 : Newtonian Mechanics

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

Example Question #261 : Newtonian Mechanics

A car traveling at  suddenly applies the brakes until it comes to a stop. If the car decelerates at a constant rate of , how long will it take the car to come to a stop?

Possible Answers:

Correct answer:

Explanation:

To answer this question, we'll first need to find the distance the car travels before coming to a stop. Since we are told in the question stem that the car is decelerating at a constant rate, we know that we have a situation in which acceleration is constant and thus we can make use of the kinematic equations.

And since we know that the car is coming to a stop, we know that our final velocity will be equal to zero.

Furthermore, since we know the car is decelerating, we know it is slowing down and is thus accelerating in the direction opposite to its direction of motion. So if we assign a positive value to the velocity, then the acceleration will have a negative value.

Now that we have found the distance traveled by the car, we can calculate the amount of time it takes for the car to come to a stop.

Plug in known values and solve.

From the above expression, we see that there are two solutions that can satisfy the value of . Only one of them, however, will make any sense physically. To solve for the two values, we will need to use the quadratic equation.

Our known values for the variables above are as follows: 

Plug into the quadratic equation and solve.

 and . The negative value makes no sense physically, thus our answer is .

An alternative (and much faster) way of solving this problem that yields the same answer is to use the following equation.

We know that our final velocity is equal to zero since the car is coming to a stop, and we're told in the question stem that we're dealing with a constant deceleration (negative acceleration).

Example Question #22 : Motion In One Dimension

A hungry wasp spots an fly wandering about. Assuming the wasp attacks the fly from behind (they are both traveling in the same direction) with speed v, and the fly is stationary, what is the speed of the wasp and fly after the collision? Assume the fly and wasp are one object after the collision. Your answer should be in terms of M, m, v where M is the mass of the wasp, m is the mass of the fly and v is the original speed of the wasp. 

Possible Answers:

, they are both stationary after the collision.

Correct answer:

Explanation:

Considering the wasp aims to eat the fly, we assume the fly and wasp are one body after the collision. This is an inelastic collision. We can solve this with conservation of momentum.

 or 

For the two body inelastic colision between the wasp and the fly, we can rewrite this as:

Then taking into account the fact the fly is stationary initially:

Then solve for the velocity of the fly and the wasp after the collision:

 

Example Question #301 : Ap Physics 1

A person travelling at a rate of , with initial position at  will have travelled to  in how much time? 

Possible Answers:

Correct answer:

Explanation:

This is a simple question of rate, time, and distance. 

, where  is distance travelled,  is rate,  is time passed. 

In our case, we know the rate  is 

We also know that the person travelled to  having originally started at  at . Keeping this in mind, the distance travelled is:

Now we just solve for time:

Example Question #262 : Newtonian Mechanics

Suppose that a ball is thrown straight upward and falls back to the ground in a time . If this same ball is thrown straight upward on a distant planet whose gravity is only one-third that of Earth's, then  will change by what factor?

Possible Answers:

Decrease by a factor of 

Increase by a factor of 

Increase by a factor of 

Decrease by a factor of 

Correct answer:

Increase by a factor of 

Explanation:

In this question, we're being asked to determine how long a ball will remain in the air when it is thrown vertically upward on a planet with reduced gravity. First, we'll need to find an expression that relates gravity with the amount of time the ball remains in the air. To do this, we can make use of the kinematics equations.

Furthermore, since we know the ball will land where it began, we know that .

Moreover, if we define the upward direction as positive and the downward direction as negative, then we know that , since gravity is always pointing in the downward direction.

The above expression is the one we're looking for because it relates time and gravity. From this expression, we can conclude that if the magnitude of gravity is reduced by a factor of three, then the time variable will increase by a factor of three.

Example Question #23 : Motion In One Dimension

Suppose that a car undergoing uniform acceleration starts from rest and travels a distance of  in a time span of . What is the acceleration that this car experiences?

Possible Answers:

Correct answer:

Explanation:

In this problem, we're told that a uniformly accelerating car travels a certain distance in a given amount of time, and we're asked to solve for the acceleration.

For starters, it's important to notice that the car is undergoing uniform acceleration. This means that the car's acceleration is constant, which is important because it means we can utilize the kinematic equations. For this problem, we'll need to use an equation that relates displacement, time, and acceleration.

Because the car is initially starting at rest, we can set the  term equal to zero.

Example Question #21 : Linear Motion And Momentum

A high jumper in track and field jumps from the ground at . How high is she able to jump? Assume gravity is .

Possible Answers:

Correct answer:

Explanation:

To solve this problem we look to the kinematic equations. Based on our need to include distance there are only two equations that might work and since we only know the initial speed we will use:

Because gravity is the only force acting to slow her upward rise we can find the time:

 

Example Question #25 : Linear Motion And Momentum

Consider a particle initially located at  and moving with initial velocity . Assuming a constant acceleration of , calculate the position at a time of .

Possible Answers:

Correct answer:

Explanation:

Looking at the initial information we are given about the particle at , we can construct the equation of motion for the position of the particle as:

Plug in our values and solve.

Example Question #261 : Newtonian Mechanics

A car is traveling on the freeway at  when the driver sees traffic stopped  ahead. What acceleration is needed to stop the car safely?

Possible Answers:

Correct answer:

Explanation:

We begin by noting we have been given no information about the time in which the stopping is to occur, only velocities and distance. This points us to the following kinematic equation:

By using the given information, and noting that coming to a stop implies a final velocity of , we can directly substitute the numbers in and solve. It makes sense the final value is negative because the car is being accelerated in the negative direction in order to stop it.

Example Question #261 : Newtonian Mechanics

 An airplane is flying at . It encounters a tailwind traveling at . At what speed is the airplane traveling?

Possible Answers:

Correct answer:

Explanation:

A tailwind is a wind traveling in the same direction as an object. The find the new speed of the airplane, we add the speed of the tailwind to the speed of the airplane.

Example Question #32 : Motion In One Dimension

A ball is dropped at time  from 20 meters above the ground. 

How long does it take for the ball to hit the ground?

Possible Answers:

Correct answer:

Explanation:

Using the following kinematic equation:

  

 is the final position, in meters, ( for ground).

 is the starting position ( above ground)

 is the starting velocity   because the ball is dropped and therefor starts from rest).

 is the acceleration due to gravity given by the problem statement.

Input these values into the first ruling equation and note that in this notation up is the positive direction so the acceleration due to gravity is negative:

Then we solve for .

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