7.4 MC Answers and Review

⛔STOP⛔ Before you look at the answers make sure you gave this practice quiz a try so you can assess your understanding of the concepts covered in Unit 7. Click here for the AP Physics C: Mechanics Multiple Choice Questions. Facts about the test: The AP Physics C: Mechanics exam has 35 multiple choice questions and you will be given 45 minutes to complete the section. That means it should take you around 15 minutes to complete 12 questions.

The following questions were not written by College Board and, although they cover information outlined in the AP Physics C: Mechanics Course and Exam Description, the formatting on the exam may be different.

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B. 1/2g

C. 2g

D. 4g

Answers: Use Newton's Law of Universal Gravitation, and remember that the force of gravity can be written as "mg" when close to the surface of the planet. It is an inverse square law, so if the radius doubles, g must decrease by a factor of 4.

📄Study AP Physics C: Mechanics, Unit 7.1: Gravitational Forces

A. The ratio of the squares of the periods of two planets is equal to the ratio of the cubes of their semi-major axes.

B. If you were to draw a line from the sun to the object in orbit, the object would sweep equal areas along the ellipse in equal amounts of time.

D. The orbits of planetary bodies are ellipses with the sun at one of the two foci of the ellipse.

Answers: The orbits of the planets are ellipses, not circles. The other three options are Kepler's Laws, and so they are all true.

📄Study AP Physics C: Mechanics, Unit 7.2: Orbits of Planets and Satellites

A. 1/25

B. 1/5

D.25

Answers: Use the relationship vArA = vBrB, which comes from the conservation of angular momentum. Rearrange to solve for vA/aB.

📄Study AP Physics C: Mechanics, Unit 7.2: Orbits of Planets and Satellites

A. At the point closest to the planet

C. At the two points halfway between the nearest and furthest points.

D. There's not enough information to tell.

Answers: This is a conservation of angular momentum question. As the rotational inertia of the system increases, the kinetic energy decreases, and vice versa. The kinetic energy is the smallest when the rotational inertia is the largest, which is at the point furthest away from the planet.

📄Study AP Physics C: Mechanics, Unit 7.2: Orbits of Planets and Satellites

A. Escape velocity is the minimum speed needed for an object to escape from the gravitational force of a massive object.

B. When an object has escaped gravity’s influence, the total energy must equal zero.

C. When an object has escaped gravity’s influence, the gravitational potential energy must equal zero.

Answers: By definition, the escape velocity is the minimum speed needed to escape a gravitational force. In order for that to happen, the total energy must equal zero (so both the kinetic and gravitational potential energies must equal zero).

📄Study AP Physics C: Mechanics, Unit 7.1: Gravitational Forces

B. 5 m/s^2

C. 10 m/s^2

D. 20 m/s^2

Answers: Use Newton's Law of Universal Gravitation, and remember that the force of gravity can be written as "mg" when close to the surface of the planet. It is an inverse square law, so if the radius doubles, g must decrease by a factor of 4.

📄Study AP Physics C: Mechanics, Unit 7.1: Gravitational Forces

B. The velocity of the satellite is directly proportional to its mass.

C. The period of the orbit is determined by the mass of the satellite.

D. The gravitational potential energy of the satellite is directly proportional to its height.

Answers: The only force acting is gravitational, and it acts as the centripetal force for the motion. The velocity of the satellite doesn't depend on its mass, nor does the period. The potential energy depends on the mass of the Earth and the satellite, and the distance between them.

📄Study AP Physics C: Mechanics, Unit 7.2: Orbits of Planets and Satellites

8. A comet follows an elliptical orbit around the Sun. At what point in its orbit does the comet sweep through the greatest area in a constant time t?

A. The comet sweeps through the same amounts of area in equal time only as it moves further from the Sun.

B. The comet sweeps through the same amount of area in equal time when it is furthest from the Sun.

C. The comet sweeps through the same amount of area in equal time when it is closest to the Sun.

Answers: This is Kepler's 2nd Law.

📄Study AP Physics C: Mechanics, Unit 7.2: Orbits of Planets and Satellites

B. GmM/4R

C. 2GmM/R

D. 4GmM/R

Answers: Use K = 1/2mv^2. Plug in the expression for the velocity of a satellite for v.

📄Study AP Physics C: Mechanics, Unit 7.2: Orbits of Planets and Satellites

A. The astronauts are not accelerating.

B. The astronauts have no mass when in space.

D. The astronauts are too far away to feel the effects of the Earth's gravity,

Answers: Earth's gravitational force acts as centripetal force causing the ISS to orbit. If there was no gravity, the ISS would travel in a straight line path away from the Earth. The astronauts still have mass, and since the Earth exerts a force on them, they are accelerating.

📄Study AP Physics C: Mechanics, Unit 7.1: Gravitational Forces

A. The tangential velocity of the second satellite is smaller than the first satellite.

C. The tangential velocity of the second satellite is equal to the first satellite.

D. There's not enough information to tell.

Answers: The period of the two satellites are the same. Use the equation for the period of an object in UCM to see that as the radius increases, the tangential velocity also increases.

📄Study AP Physics C: Mechanics, Unit 7.2: Orbits of Planets and Satellites

A. A 2 kg mass and a 3 kg mass are held 1 meter apart.

C. A 2 kg mass and a 10 kg mass are held 2 meters apart.

D. Two 4 kg masses are held 2 meters apart.

Answers: Use Newton's Law of Universal Gravitation and plug in the values. Remember that you can leave "G" as a constant, since it will be the same for all of the situations.

📄Study AP Physics C: Mechanics, Unit 7.1: Gravitational Forces

A. v/2

B. 2v

D. v/sqrt(2)

Answers: Use the equation for the speed of a satellite to solve. Compare the initial equation with the equation using 2M.

📄Study AP Physics C: Mechanics, Unit 7.2: Orbits of Planets and Satellites

A. The centripetal acceleration of the satellite depends on the value of R.

B. The angular velocity of the satellite depends on the value of R.

C. The magnitude of the centripetal force on the satellite is constant.

Answers: The satellite is moving in uniform circular motion, so while the speed of the satellite is constant, its direction is changing. Therefore the tangential velocity is not constant. This was a tricky one! (maybe)

📄Study AP Physics C: Mechanics, Unit 7.2: Orbits of Planets and Satellites

B. -GmMe / 4Re^2

C. -2GmMe / Re

D. -4GmMe / Re^2

Answers: Use the equation for gravitational potential energy from the equation sheet. Plug in m for the mass of the satellite, Me for the mass of the Earth, and Re for the radius of the Earth.

📄Study AP Physics C: Mechanics, Unit 7.1: Gravitational Forces

Time to Check Your Answers on Unit 1 Practice Questions Answers and Review! 🙌