High School Physics : Linear Motion

Study concepts, example questions & explanations for High School Physics

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Example Questions

Example Question #5 : Understanding Motion In Two Dimensions

A ball rolls off of a table with an initial horizontal velocity of \displaystyle 19\frac{m}{s}. If the table is \displaystyle 3m high, what is the final vertical velocity?

\displaystyle \small g=-9.8\frac{m}{s^2}

Possible Answers:

\displaystyle 19.6\frac{m}{s}

\displaystyle -7.67\frac{m}{s}

\displaystyle 15.34\frac{m}{s}

\displaystyle -58.8\frac{m}{s}

\displaystyle 3.83\frac{m}{s}

Correct answer:

\displaystyle -7.67\frac{m}{s}

Explanation:

The problem states that the initial velocity is only in the horizontal direction; the initial vertical velocity is zero. We now know initial velocity, acceleration, and distance traveled.

Remember, even though the distance it will travel is \displaystyle 3m, its displacement will be \displaystyle -3m as it moves in the downward direction.

Using these values and the appropriate motion equation, we can solve for the final velocity. The best equation to use is:

\displaystyle v_{fy}^2=v_{iy}^2+2a\Delta y

Use the given values to find the final velocity.

\displaystyle v_{fy}^2=0\frac{m}{s}^2 +2(-9.8\frac{m}{s^2})(-3m)

\displaystyle v_{fy}^2=58.8\frac{m^2}{s^2}

\displaystyle \sqrt{v_{fy}^2}}=\sqrt{58.8\frac{m^2}{s^2}}

\displaystyle v_{fy}=7.67\frac{m}{s}

Because we just took the square root of a number, we got an absolute value for our \displaystyle v_{fy}; however, velocity is a vector and can be either positive or negative depending on direction. Because the ball is headed downward, the final velocity should correctly be \displaystyle -7.67\frac{m}{s}. Remember that a negative number squared gives a positive value, just like a positive number.

Example Question #6 : Understanding Motion In Two Dimensions

A ball rolls off of a table with an initial horizontal velocity of \displaystyle 19\frac{m}{s}. If the table is \displaystyle 3m high, how far from the table will it land?

\displaystyle \small g=-9.8\frac{m}{s^2}

Possible Answers:

\displaystyle 19m

\displaystyle 14.86m

\displaystyle 0.782m

\displaystyle 7.43m

\displaystyle 29.72m

Correct answer:

\displaystyle 14.86m

Explanation:

We can solve for the horizontal distance using only the horizontal velocity: \displaystyle \Delta x =v_xt.

We are given the value of \displaystyle v_x, but we need to find the time. Time in the air will be determined by the vertical components of the ball's motion.

We know the height of the table, the initial velocity, and gravity. Using these values with the appropriate motion equation, we can solve for the time.

The best equation to use is:

\displaystyle \Delta y =v_it+\frac{1}{2}at^2

We can use our values to solve for the time. Keep in mind that the displacement will be negative because the ball is traveling in the downward direction!

\displaystyle -3m=0\frac{m}{s}t+\frac{1}{2}(-9.8\frac{m}{s^2})t^2

\displaystyle -3m=\frac{1}{2}(-9.8\frac{m}{s^2})t^2

\displaystyle -3m=(-4.9\frac{m}{s^2})t^2

\displaystyle \frac{-3m}{-4.9\frac{m}{s^2}}=t^2

\displaystyle 0.612s^2=t^2

\displaystyle \sqrt{0.612s^2}=\sqrt{t^2}

\displaystyle 0.782s=t

Now we have both the time and the horizontal velocity. Use the original equation to solve for the distance.

\displaystyle \Delta x =v_xt

\displaystyle \Delta x =(19\frac{m}{s})(0.782s)

\displaystyle \Delta x =14.86m

Example Question #1 : Understanding Motion In Two Dimensions

A man stands on a tall ladder of height \displaystyle h. He leans over a little too far and falls off the ladder. What would be the best way to describe his fall?

Possible Answers:

Parabolic motion

Circular motion

We would need to know air resistance in order to determine his type of motion

One-dimensional motion

We would need to know his mass in order to determine the type of motion

Correct answer:

Parabolic motion

Explanation:

The man's fall will be parabolic as there will be both horizontal and vertical components. His vertical component of the fall will be standard free-fall caused by his acceleration due to gravity. His horizontal component of the fall will come from him "leaning too far" in one direction. Even a small horizontal velocity will create a horizontal trajectory.

This is why when people lean and fall off of ladders they either try to grab onto the ladder (try to negate their horizontal velocity) or fall a small distance away from the base of the ladder.

Example Question #8 : Understanding Motion In Two Dimensions

A cannon on level ground fires a cannon ball at \displaystyle 250\frac{m}{s} at \displaystyle 30^\circ above the horizontal. What is the final horizontal velocity?

Possible Answers:

\displaystyle 216.5\frac{m}{s}

\displaystyle 250\frac{m}{s}

\displaystyle 125\frac{m}{s}

\displaystyle 38.56\frac{m}{s}

\displaystyle 247\frac{m}{s}

Correct answer:

\displaystyle 216.5\frac{m}{s}

Explanation:

Remember that the velocity in the horizontal direction stays constant through the projectile's motion. There is no force in the horizontal direction, only in the vertical direction. That means the initial and final horizontal velocities will be the same.

To find our \displaystyle v_x, we need to use cosine trigonometry, with the horizontal velocity as the adjacent side and the total initial velocity as the hypotenuse.

\displaystyle \cos(\theta)=\frac{adj}{hyp}=\frac{v_x}{v_i}

\displaystyle v_i*\cos(\theta)=v_x

\displaystyle 250\frac{m}{s}*\cos(30^\circ)=v_x

\displaystyle 216.5\frac{m}s=v_x

Example Question #9 : Understanding Motion In Two Dimensions

A cannon on level ground fires a cannon ball at \displaystyle 250\frac{m}{s} at \displaystyle 30^\circ above the horizontal. What is the initial vertical velocity?

Possible Answers:

\displaystyle 125\frac{m}{s}

\displaystyle 144.3\frac{m}{s}

\displaystyle -247\frac{m}{s}

\displaystyle 216.5\frac{m}{s}

\displaystyle 38.6\frac{m}{s}

Correct answer:

\displaystyle 125\frac{m}{s}

Explanation:

We are given the total initial velocity and the angle of the initial trajectory. Using these values, we can use trigonometry to solve for the initial vertical velocity.

We will need to use sine, with the vertical velocity as the opposite side and the total velocity as the hypotenuse.

\displaystyle \sin(\theta)=\frac{opp}{hyp}=\frac{v_y}{v_i}

\displaystyle v_i*\sin(\theta)=v_y

\displaystyle 250\frac{m}{s}*\sin(30^\circ)=v_y

\displaystyle 125\frac{m}s=v_y

Example Question #10 : Understanding Motion In Two Dimensions

A cannon on level ground fires a cannon ball at \displaystyle 250\frac{m}{s} at \displaystyle 30^\circ above the horizontal. How high does the cannon ball go?

Possible Answers:

\displaystyle 2391.6m

\displaystyle 920.6m

\displaystyle 7997.2m

\displaystyle 797.2m

\displaystyle 823.3m

Correct answer:

\displaystyle 797.2m

Explanation:

To find the height of the projectile, we can use the appropriate kinematics equation:

\displaystyle v_f^2=v_i^2+2a\Delta y

We know that the final velocity at the maximum height will be zero, and we also know the acceleration due to gravity. Before we can use the equation, however, we must solve for the initial vertical velocity. We are given the total initial velocity and the angle of the initial trajectory. Using these values, we can use trigonometry to solve for the initial vertical velocity.

\displaystyle v_i*\sin(\theta)=v_y

\displaystyle 250\frac{m}{s}*\sin(30^\circ)=v_y

\displaystyle 125\frac{m}s=v_y

Now that we know the initial vertical velocity, we can return to the kinematics equation to solve for the final displacement.

\displaystyle v_f^2=v_i^2+2a\Delta y

\displaystyle (0\frac{m}{s})^2=(125\frac{m}{s})^2+2(-9.8\frac{m}{s^2})\Delta y

\displaystyle 0=15625\frac{m^2}{s^2}+(-19.6\frac{m}{s^2})\Delta y

\displaystyle -15625\frac{m^2}{s^2}=-19.6\frac{m}{s^2}*\Delta y

\displaystyle \frac{-15625\frac{m^2}{s^2}}{-19.6\frac{m}{s^2}}=\Delta y

\displaystyle 797.2m=\Delta y

Example Question #41 : Linear Motion

A cannon on level ground fires a cannon ball at \displaystyle 250\frac{m}{s} at \displaystyle 30^\circ above the horizontal. How long does it take the ball to reach its maximum height?

Possible Answers:

\displaystyle 25.5s

\displaystyle 6.38s

\displaystyle 12.76s

\displaystyle 22.04s

\displaystyle 797s

Correct answer:

\displaystyle 12.76s

Explanation:

To find the time for the projectile to reach its maximum height, we can use the appropriate kinematics equation:

\displaystyle v_f=v_i+at

We know that the final velocity at the maximum height will be zero, and we also know the acceleration due to gravity. Before we can use the equation, however, we must solve for the initial vertical velocity. We are given the total initial velocity and the angle of the initial trajectory. Using these values, we can use trigonometry to solve for the initial vertical velocity.

\displaystyle v_i*\sin(\theta)=v_y

\displaystyle 250\frac{m}{s}*\sin(30^\circ)=v_y

\displaystyle 125\frac{m}s=v_y

Now that we know the initial vertical velocity, we can return to the kinematics equation to solve for the time.

\displaystyle v_{fy}=v_{iy}+at

\displaystyle 0\frac{m}{s}=125\frac{m}{s}+(-9.8\frac{m}{s^2})t

\displaystyle -125\frac{m}{s}=-9.8\frac{m}{s^2}t

\displaystyle \frac{-125\frac{m}{s}}{-9.8\frac{m}{s^2}}=t

\displaystyle 12.76s=t

Example Question #42 : Linear Motion

A cannon on level ground fires a cannon ball at \displaystyle 250\frac{m}{s} at \displaystyle 30^\circ above the horizontal. What is the total time the ball is in the air?

Possible Answers:

\displaystyle 12.76s

\displaystyle 44.18s

\displaystyle 25.52s

\displaystyle 125s

\displaystyle 22.09s

Correct answer:

\displaystyle 25.52s

Explanation:

To find the total time, we can solve for the time to reach the peak and then double this value to find the total time.

To find the time for the projectile to reach its maximum height, we can use the appropriate kinematics equation:

\displaystyle v_f=v_i+at

We know that the final velocity at the maximum height will be zero, and we also know the acceleration due to gravity. Before we can use the equation, however, we must solve for the initial vertical velocity. We are given the total initial velocity and the angle of the initial trajectory. Using these values, we can use trigonometry to solve for the initial vertical velocity.

\displaystyle v_i*\sin(\theta)=v_y

\displaystyle 250\frac{m}{s}*\sin(30^\circ)=v_y

\displaystyle 125\frac{m}s=v_y

Now that we know the initial vertical velocity, we can return to the kinematics equation to solve for the time.

\displaystyle v_{fy}=v_{iy}+at

\displaystyle 0\frac{m}{s}=125\frac{m}{s}+(-9.8\frac{m}{s^2})t

\displaystyle -125\frac{m}{s}=-9.8\frac{m}{s^2}t

\displaystyle \frac{-125\frac{m}{s}}{-9.8\frac{m}{s^2}}=t

\displaystyle 12.76s=t

Double this value to find the total flight time.

\displaystyle 2(12.76s)=25.5s

Example Question #43 : Linear Motion

A cannon on level ground fires a cannon ball at \displaystyle 250\frac{m}{s} at \displaystyle 30^\circ above the horizontal. What is the total horizontal distance that the cannon ball travels?

Possible Answers:

\displaystyle 2762.54m

\displaystyle 52.56m

\displaystyle 3190m

\displaystyle 74.33m

\displaystyle 5525.08m

Correct answer:

\displaystyle 5525.08m

Explanation:

To solve for the horizontal distance we can use the simple formula for velocity, since there is no acceleration in the horizontal direction.

\displaystyle \Delta x =v_x*t

This leaves us two distinct variables: \displaystyle v_x and \displaystyle t.

To find our \displaystyle v_x, we need to use cosine.

\displaystyle v_i*\cos(\theta)=v_x

\displaystyle 250\frac{m}{s}*\cos(30^\circ)=v_x

\displaystyle 216.5\frac{m}s=v_x

To find the time, we can solve for the time to reach the peak and then double this value to find the total time.

To find the time for the projectile to reach its maximum height, we can use the appropriate kinematics equation:

\displaystyle v_f=v_i+at

We know that the final velocity at the maximum height will be zero, and we also know the acceleration due to gravity. Before we can use the equation, however, we must solve for the initial vertical velocity. We are given the total initial velocity and the angle of the initial trajectory. Using these values, we can use trigonometry to solve for the initial vertical velocity.

\displaystyle v_i*\sin(\theta)=v_y

\displaystyle 250\frac{m}{s}*\sin(30^\circ)=v_y

\displaystyle 125\frac{m}s=v_y

Now that we know the initial vertical velocity, we can return to the kinematics equation to solve for the time.

\displaystyle v_{fy}=v_{iy}+at

\displaystyle 0\frac{m}{s}=125\frac{m}{s}+(-9.8\frac{m}{s^2})t

\displaystyle -125\frac{m}{s}=-9.8\frac{m}{s^2}t

\displaystyle \frac{-125\frac{m}{s}}{-9.8\frac{m}{s^2}}=t

\displaystyle 12.76s=t

Double this value to find the total flight time.

\displaystyle 2(12.76s)=25.5s

Now that we know our total time and horizontal velocity, we can return to the velocity equation to solve for the distance traveled.

\displaystyle \Delta x =v_x*t

\displaystyle \Delta x =216.5\frac{m}{s}*25.52s

\displaystyle \Delta x =5525.08m

Example Question #44 : Linear Motion

Peter walks \displaystyle 30.7m \text{ at }38.3^{\circ} north of east. What is his total displacement along the y-axis?

Possible Answers:

\displaystyle 23.93m

\displaystyle 19.03m\ \text{north}

\displaystyle 19.03m \ \text{east}

\displaystyle 19.03m

\displaystyle 23.93m\ \text{north}

Correct answer:

\displaystyle 19.03m\ \text{north}

Explanation:

Start by drawing a picture.

Screen_shot_2013-12-11_at_9.26.55_am

By traveling both north and east, Peter has displacement along both the x-axis and the y-axis. Recognize that this makes a right triangle.

We can solve for the y-axis displacement by using trigonometry.

\displaystyle \sin(\Theta)=\frac{\text{opposite}}{\text{hypotenuse}}

In this case, we know the angle and the hypotenuse. The y-displacement is opposite the angle.

\displaystyle \sin(38.3^o)=\frac{Y}{30.7m}

Multiply both sides by \displaystyle 30.7 and solve.

\displaystyle (30.7m)\sin(38.3^o)=Y

\displaystyle 19.03m=Y

Since we are solving for the displacement, we need to include the direction. The displacement is \displaystyle \small 19.03m\ \text{north}.

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