College Physics : Motion

Study concepts, example questions & explanations for College Physics

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Example Questions

Example Question #1 : Harmonic Motion

If the mass of a simple pendulum is quadrupled, then its period __________.

Possible Answers:

quadruples

is reduce to one quarter

doubles

remains the same

Correct answer:

remains the same

Explanation:

We know that the equation for the period of a simple pendulum is \(\displaystyle T = 2\pi\sqrt{\frac{L}{g}}\). This equation does not depend on mass. It is only affected by the length of the pendulum (L) and the gravitational constant (g). Therefore, adding mass to the pendulum will not effect the period, so the period remains the same.

Example Question #2 : Harmonic Motion

A violin string \(\displaystyle 37.0 cm\) long has a linear density of \(\displaystyle 2.9\frac{g}{m}\). What is the string tension if the second harmonic has a frequency of \(\displaystyle 770 Hz\)?

Possible Answers:

\(\displaystyle 312 N\)

\(\displaystyle 1024 N\)

\(\displaystyle 373N\)

\(\displaystyle 77 N\)

Correct answer:

\(\displaystyle 373N\)

Explanation:

Since we are solving for string tension, we need to use the frequency equation with the tension variable in it. That equation is \(\displaystyle f = \frac{n}{2L}\sqrt\frac{T}{\mu}\) where \(\displaystyle f\) is the frequency, \(\displaystyle n\) is the number of the harmonic, \(\displaystyle L\) is the length of the string, \(\displaystyle \mu\) is the linear density of the string, and \(\displaystyle T\) is the tension of the string.

We are given:

\(\displaystyle n=2\)

\(\displaystyle L=37.0 cm\)

\(\displaystyle f=770 Hz\)

\(\displaystyle \mu=2.9\frac{g}{m}\)

Next we must convert the length of \(\displaystyle 37.0 cm\) to meters which is \(\displaystyle 0.37m\) and the mass density of \(\displaystyle 2.9\frac{g}{m}\) to \(\displaystyle 0.0029\frac{kg}{m}\). Then we plug in our known values into the equation and solve for the string tension. The result is \(\displaystyle 373N\).

Example Question #1 : Harmonic Motion

For a simple harmonic motion governed by Hooke's Law, \(\displaystyle F =-kx\), if \(\displaystyle T\) is the period then the quantity \(\displaystyle \frac{T}{2\pi}\) is equivalent to which of the following?

Possible Answers:

\(\displaystyle \frac{k}{m}\)

\(\displaystyle \sqrt{\frac{m}{k}}\)

\(\displaystyle \sqrt{\frac{k}{m}}\)

\(\displaystyle \frac{m}{k}\)

Correct answer:

\(\displaystyle \sqrt{\frac{m}{k}}\)

Explanation:

We know that T is the period. The equation for T is \(\displaystyle T = 2\pi\sqrt{\frac{m}{k}}\) for harmonic motion.

Solve for \(\displaystyle \frac{T}{2\pi}\) by dividing the equation by \(\displaystyle 2\pi\) on both sides. The result is \(\displaystyle \sqrt{\frac{m}{k}}\), which is the answer.

Example Question #4 : Harmonic Motion

A pendulum on earth has a period of \(\displaystyle 1.82s\). What is it's period on Mars with it's gravity is \(\displaystyle 3.71 \frac{m}{s^2}?\)

Possible Answers:

\(\displaystyle 2.96s\)

\(\displaystyle 1.63s\)

\(\displaystyle 2.23s\)

\(\displaystyle 1.47s\)

Correct answer:

\(\displaystyle 2.96s\)

Explanation:

First, we need to find the length of the pendulum. Begin with the equation for finding the period of a pendulum:

\(\displaystyle T = 2\pi \sqrt\frac{L}{g}\)

solve for \(\displaystyle L\) to get:

\(\displaystyle L = \frac{T^2g}{4\pi^2}\)

Now we can plug in our given values:

\(\displaystyle L = \frac{(1.82)^2(9.81)}{4\pi^2} = 0.823m\)

Since we have the length of the pendulum determined, we can now find the period of the pendulum on Mars:

\(\displaystyle T = 2\pi\sqrt\frac{L}{g} = 2\pi \sqrt\frac{0.823}{3.71}= 2.96 s\)

Example Question #31 : College Physics

The bases on a baseball field are 90 feet apart.

A players hits a home run and gets around the bases in 20 seconds what is the players total speed?

Possible Answers:

\(\displaystyle 0\frac{ft}{s}\)

\(\displaystyle 14\frac{ft}{s}\)

\(\displaystyle 22\frac{ft}{s}\)

\(\displaystyle 18\frac{ft}{s}\)

\(\displaystyle 20\frac{ft}{s}\)

Correct answer:

\(\displaystyle 18\frac{ft}{s}\)

Explanation:

\(\displaystyle speed= \frac{distance}{time}\)

 

\(\displaystyle Distance = 360ft\)

 

\(\displaystyle Time= 20s\)

 

\(\displaystyle \frac{360ft}{20s}= 18\frac{ft}{s}\)

Example Question #1 : Motion Diagrams

The bases on a baseball field are \(\displaystyle 90ft\) apart in a perfect square.

A player hits a home run and goes around all 4 bases. What is the total distance he travelled?

Possible Answers:

\(\displaystyle 270ft\)

\(\displaystyle 360ft\)

\(\displaystyle 450ft\)

\(\displaystyle 0ft\)

\(\displaystyle 180ft\)

Correct answer:

\(\displaystyle 360ft\)

Explanation:

There is 90 feet between each base and distance does not depend on direction. Distance is scaler therefore, the batter travels 360 feet.

Example Question #2 : Motion Diagrams

The bases on a baseball field are 90 feet apart.

A players hits a home run and gets around the bases in 20 seconds what is the players total velocity?

Possible Answers:

\(\displaystyle 18\frac{ft}{s}\)

\(\displaystyle 0\frac{ft}{s}\)

\(\displaystyle 20\frac{ft}{s}\)

\(\displaystyle 36\frac{ft}{s}\)

\(\displaystyle 15\frac{ft}{s}\)

Correct answer:

\(\displaystyle 0\frac{ft}{s}\)

Explanation:

\(\displaystyle Velocity = \frac{displacement}{time}\)

Displacement is a vector. Magnitude and direction matter.

\(\displaystyle Displace = 0 \rightarrow Velocity= 0\)

Example Question #3 : Motion Diagrams

The bases on a baseball field are \(\displaystyle 90ft\) apart.

A player hits a home run and runs around all four bases. What is his total displacement?

Possible Answers:

\(\displaystyle 270ft\)

\(\displaystyle 0ft\)

\(\displaystyle 90ft\)

\(\displaystyle 360ft\)

\(\displaystyle 180ft\)

Correct answer:

\(\displaystyle 0ft\)

Explanation:

Displacement is a vector. Therefore, magnitude and direction matters and because direction matters the total displacement is 0 feet.

Example Question #4 : Motion Diagrams

\(\displaystyle 2kg\) ball is dropped and it hits the floor is \(\displaystyle 4 s\). How high was the ball dropped from?

\(\displaystyle g=10\frac{m}{s^2}\)

Possible Answers:

\(\displaystyle 120 m\)

\(\displaystyle 80 ft\)

\(\displaystyle 160 m\)

\(\displaystyle 80 m\)

\(\displaystyle 100 m\)

Correct answer:

\(\displaystyle 80 m\)

Explanation:

We are dealing with motion in one plane and the initial velocity is zero. We must use the following kinematics equation to plug in known values and solve:

\(\displaystyle d = \frac{1}{2}at^2\)

\(\displaystyle d = \frac{1}{2}(10\frac{m}{s^2})(4s^2)\)

\(\displaystyle d = 80m\)

Example Question #5 : Motion Diagrams

There are \(\displaystyle 90 ft\) between the bases on a baseball field and they make a square.

Jim hits a double and reaches second base in \(\displaystyle 12 s\). What is his velocity?

Possible Answers:

\(\displaystyle 3\frac{ft}{s}\)

\(\displaystyle 90\frac{ft}{s}\)

\(\displaystyle 15\frac{ft}{s}\)

\(\displaystyle 30\frac{ft}{s}\)

\(\displaystyle 21.2\frac{ft}{s}\)

Correct answer:

\(\displaystyle 21.2\frac{ft}{s}\)

Explanation:

\(\displaystyle Speed=\frac{Distance}{Time}\)

\(\displaystyle Speed=\frac{180ft}{12s}=15\frac{ft}{s}\) 

Velocity is a vector so direction matters. Therefore this creates a right triangle with the two isosceles side having a velocity of \(\displaystyle 15\frac{ft}{s}\). Therefore, the hypotenuse is \(\displaystyle 21.2\frac{ft}{s}\)when you perform Pythagorean theorem.

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