AP Physics B : AP Physics B

Study concepts, example questions & explanations for AP Physics B

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Example Questions

Example Question #1 : Motion In One Dimension

If a 15kg ball takes five seconds to strike the ground when released from rest, at what height was the ball dropped? 

Possible Answers:

75m

50m

100m

250m

125m

Correct answer:

125m

Explanation:

Using the equation \(\displaystyle d=v_ot + (1/2)a(t^2)\) we can find the distance at which the ball was dropped. Notice that the mass of the ball does not matter in this problem. We are told that the ball is dropped from rest making, \(\displaystyle v_o =0\), thus we have \(\displaystyle d=(1/2)a(t^2)\). When we plug in our values, and assuming that acceleration is equal to gravity (10m/s2) we find that  \(\displaystyle d=(1/2)(10)(5^2)\) = 125m.

Example Question #1 : Motion In One Dimension

How far will an object travel after ten seconds if it is dropped into a bottomless pit? 

Possible Answers:

300m

50m

25m

500m

250m

Correct answer:

500m

Explanation:

Since the object is dropped, the inital velocity is zero. Gravity is the only acceleration, the time is ten seconds, and the distance at which the object travels is unknown.

The equation \(\displaystyle d=v_ot +(1/2)at^2\) can be used to find the distance traveled.

\(\displaystyle d=0+(1/2)(10m/s^2)(10s)^2=500m\)

Example Question #1 : Motion In One Dimension

How long does it take an object to travel a distance of 30m from rest at a constant acceleration of 2m/s2?

Possible Answers:

\(\displaystyle 10.9s\)

\(\displaystyle 12.6s\)

\(\displaystyle 2.7s\)

\(\displaystyle 5.5s\)

\(\displaystyle 7.3s\)

Correct answer:

\(\displaystyle 5.5s\)

Explanation:

Using the equation \(\displaystyle d=v_ot + (1/2)at^2\), we can solve for time.

Since the object started at rest, \(\displaystyle v_0=0\). Now we are left with the equation \(\displaystyle d=(1/2)at^2\).

\(\displaystyle 30m=\frac{1}{2}(2\frac{m}{s^2})t^2\)

\(\displaystyle 30s^2=t^2\)

\(\displaystyle t=5.5s\)

Plugging in the remaining values we can find that t = 5.5s.

Example Question #2 : Motion In One Dimension

A person on top of a \(\displaystyle 200m\) tall building drops a rock. How long will it take for the rock to reach the ground? Ignore air resistance.

Possible Answers:

\(\displaystyle t=40.8s\)

\(\displaystyle t=6.4s\)

\(\displaystyle t=7.3s\)

\(\displaystyle t=113.4s\)

\(\displaystyle t=55.7s\)

Correct answer:

\(\displaystyle t=6.4s\)

Explanation:

Use the following kinematic equation: \(\displaystyle y=y_o+v_ot+\frac{1}{2}at^2\).

We can choose the ground to be the zero distance so that \(\displaystyle y=0\) and \(\displaystyle y_o=200\ \text{m}\).

Also, the initial speed is zero.

\(\displaystyle v_o=0\)

The kinematic equation simplifies using these values.

\(\displaystyle 0=y_o+\frac{1}{2}at^2\)

Solve to isolate the time variable.

\(\displaystyle t=\sqrt{\frac{-2y_o}{a}}\)

We know that the acceleration is the acceleration due to gravity. Now we can plug in the known values and solve.

\(\displaystyle t=\sqrt{\frac{-2(200\ \text{m})}{-9.8\ \frac{\text{m}}{\text{s}^2}}}\)

\(\displaystyle t=\sqrt{40.8s^2}=6.4s\)

Example Question #2 : Motion In One Dimension

You are driving at a speed of \(\displaystyle 20\frac{m}{s}\) and suddenly, a tree falls down on the road blocking your path. You slam on your brakes to avoid hitting the fallen tree and thus, come to a complete stop. You were at a distance of \(\displaystyle 28m\) away from the tree when you hit the brakes. Assuming that your vehicle does not skid, what is the minimum deceleration needed to avoid hitting the fallen tree?

Possible Answers:

\(\displaystyle a=-13.97\ \frac{\text{m}}{\text{s}^2}\)

\(\displaystyle a=-4.51\ \frac{\text{m}}{\text{s}^2}\)

\(\displaystyle a=0\ \frac{\text{m}}{\text{s}^2}\)

\(\displaystyle a=-7.14\ \frac{\text{m}}{\text{s}^2}\)

\(\displaystyle a=-5.12\ \frac{\text{m}}{\text{s}^2}\)

Correct answer:

\(\displaystyle a=-7.14\ \frac{\text{m}}{\text{s}^2}\)

Explanation:

Use the following kinematic equation where the initial velocity is \(\displaystyle v_o\), final velocity is \(\displaystyle v_f\), and the distance traveled is \(\displaystyle \Delta x\).

\(\displaystyle v_f^2=v_o^2+2a\Delta x\)

We can use the values in the question to solve for the acceleration.

\(\displaystyle (0\frac{m}{s})^2=(20\frac{m}{s})^2+2a(28m)\)

We rearrange the equation to solve for the acceleration.

\(\displaystyle a=\frac{0^2-(20\frac{m}{s})^2}{2(28\ \text{m})}\)

\(\displaystyle a=-7.14\ \frac{\text{m}}{\text{s}^2}\)

 

Example Question #1 : Calculating Motion In One Dimension

A \(\displaystyle 2.5kg\) is dropped from a height of \(\displaystyle 20m\). A picture is taken when the ball is \(\displaystyle 5m\) from the ground with an exposure time of \(\displaystyle 0.005s\). If the actual diameter of the ball is \(\displaystyle 15cm\), what will the vertical diameter of the ball appear to be in the picture?

Possible Answers:

\(\displaystyle 23.6cm\)

\(\displaystyle 36.8cm\)

\(\displaystyle 19.9cm\)

\(\displaystyle 17.2cm\)

\(\displaystyle 15cm\)

Correct answer:

\(\displaystyle 23.6cm\)

Explanation:

The first step to solving this problem will be to find the velocity of the ball at the point when the picture is taken. We know the initial velocity of the ball (zero), the displacement, and the acceleration. Using the appropriate kinematics formula, we can solve for the final velocity.

\(\displaystyle v_f^2=v_o^2+2a\Delta y\)

\(\displaystyle v_f^2=(0\frac{m}{s})^2+2(-9.8\frac{m}{s^2})(5m-20m)\)

\(\displaystyle v_f^2=294\frac{m^2}{s^2}\)

\(\displaystyle v_f=17.15\frac{m}{s}\)

Now that we know how fast the ball is traveling when the picture is taken, we can find the distance it travels while the shutter is open. This distance will become a motion blur, making the vertical diameter of the ball appear stretched.

\(\displaystyle v=\frac{d}{t}\rightarrow d=vt\)

\(\displaystyle d=(17.15\frac{m}{s})(0.005s)=0.086m\)

During the exposure period, the ball will travel \(\displaystyle 0.086m\), or \(\displaystyle 8.6cm\). The diameter of the ball in the vertical direction will appear to be distorted by this distance.

\(\displaystyle 15cm+8.6cm=23.6cm\)

 

Example Question #4 : Motion In One Dimension

A person stands on the edge of a straight \(\displaystyle 20 m\)-high cliff and holds a ball over the edge. The person tosses the ball directly upward with an initial speed of \(\displaystyle 5 \frac{m}{s}\). How long will it take the ball to hit the ground at the base of the cliff, \(\displaystyle 20 m\) below?

Assume \(\displaystyle 10 \frac{m}{s^2}\) for gravitational acceleration.

Possible Answers:

\(\displaystyle 43.00 s\)

\(\displaystyle 6.433 s\)

\(\displaystyle 2.562 s\)

\(\displaystyle 5.334 s\)

\(\displaystyle 16.12 s\)

Correct answer:

\(\displaystyle 2.562 s\)

Explanation:

This is projectile motion in the vertical direction only, subject to the equation of motion: \(\displaystyle (x-x_o) = v_ot + \frac{1}{2}at^2\).

For this discussion, one can define the downward direction as negative. For projectile motion, \(\displaystyle a = -10 \frac{m}{s^2}\) (gravitational acceleration, or \(\displaystyle g\)).

In this case, the ball ends up \(\displaystyle 20 m\) below where is started, so \(\displaystyle (x - x_o) = -20 m\).

The initial velocity, \(\displaystyle v_o\), is \(\displaystyle 5 \frac{m}{s}\) (upward, thus positive).

With all this, the projectile motion equation becomes:

\(\displaystyle - 20 m = (5 \frac{m}{s})t - \frac{1}{2}(10 \frac{m}{s^2})t^2\)

This can be solved for \(\displaystyle t\) using the quadratic formula:

 \(\displaystyle t = \frac{-b \pm \sqrt{b^{2}-4ac)}) }{2a}\)

 The result is:

\(\displaystyle t = \frac{5\pm 20.62}{10} = 2.562 s\) (or \(\displaystyle -1.562 s\)).

Only the positive answer option is physically possible, and is thus our correct answer.

Example Question #11 : Ap Physics B

A block is placed at a height of \(\displaystyle 18m\) on a ramp at an incline of \(\displaystyle 24^o\). It slides down from rest with an acceleration of \(\displaystyle 2.2\frac{m}{s^2}\) due to a frictional force. What is the velocity of the block when it reaches the bottom of incline?

24_deg._incline_block

Possible Answers:

\(\displaystyle v_f=12\frac{m}{s}\)

\(\displaystyle v_f=14\frac{m}{s}\)

\(\displaystyle v_f=8.9\frac{m}{s}\)

\(\displaystyle v_f=5.1\frac{m}{s}\)

\(\displaystyle v_f=16\frac{m}{s}\)

Correct answer:

\(\displaystyle v_f=14\frac{m}{s}\)

Explanation:

To find the final velocity we use the following kinematic equation.

\(\displaystyle v_f^2=v_o^2+2a\Delta x\)

The block starts from rest, so \(\displaystyle v_o=0\). The incline is eighteen meters high, so we can find the distance (hypotenuse) as follows:

\(\displaystyle \Delta x=\frac{18m}{sin24^\circ}\)

\(\displaystyle \Delta x = 44.25m\)

We know that the acceleration is \(\displaystyle a=2.2\frac{m}{s^2}\).

Plug in the values to find the velocity at the bottom of the incline.

\(\displaystyle v_f^2=(0\frac{m}{s})^2+2(2.2\frac{m}{s^2})(44.25m)\)

\(\displaystyle v_f=\sqrt{194.7\frac{m^2}{s^2}}=14\frac{m}{s}\)

Example Question #1 : Motion In Two Dimensions

An object is shot from the ground at 75m/s at an angle of 45⁰ above the horizontal. How high does the object get before beginning its descent?

Possible Answers:

420m

280m

70m

140m

Correct answer:

140m

Explanation:

The velocity must be broken down into x (horizontal) and y (vertical) components. We can use the y component to find how high the object gets. To find vertical velocity, vy, use \(\displaystyle v_y = v_o sin\theta\).

\(\displaystyle v_y = (75 \frac{m}{s})sin 45^o = 53 \frac{m}{s}\)

Next we find how long it takes to reach the top of its trajectory using \(\displaystyle v_f = v_o +at\).

\(\displaystyle 0 \frac{m}{s} = 53 \frac{m}{s} + (-10 \frac{m}{s^2})t\)

 t = 5.3s

Finally, find how high the object goes with \(\displaystyle d = v_ot +\frac{1}{2}at^{2}\).

\(\displaystyle d = (53\frac{m}{s})5.3s + \frac{1}{2}(-10\frac{m}{s^2})(5.3s)^{2} = 140 m\) 

Example Question #2 : Motion In Two Dimensions

An object is shot from the ground at 125m/s at an angle of 30o above the horizontal. How far away does the object land?

Possible Answers:

1350m

62.5m

675m

250m

Correct answer:

1350m

Explanation:

First, find the horizontal (x) and vertical (y) components of the velocity

\(\displaystyle v_x = v_o cos\theta\)

\(\displaystyle v_x = (125 \frac{m}{s}) cos(30^o) = 108\frac{m}{s}\)

\(\displaystyle v_y = v_o sin\theta\)

\(\displaystyle v_y = (125\frac{m}{s})sin(30^o) = 62.5\frac{m}{s}\)

Next, find how long the object is in the air by calculating the time it takes it to reach the top of its path, and doubling that number.

\(\displaystyle v_f = v_o + at\)

\(\displaystyle 0 \frac{m}{s} = 62.5 \frac{m}{s} + (-10 \frac{m}{s^2})t\)

t = 6.25s 

Total time in the air is therefore 12.5s (twice this value).

Finally, find distance traveled my multiplying horizontal velocity and time.

\(\displaystyle d_x = v_x(t)\)

\(\displaystyle d_x = 108 \frac{m}{s} (12.5s) = 1,350 m\)

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