3.10 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 3. Click here for the practice questions: AP Physics 1 Unit 3 Multiple Choice Questions. Facts about the test: The AP Physics 1 exam has 50 multiple choice questions (45 single-select and 5 multiple-select) and you will be given 90 minutes to complete the section. That means it should take you around 15 minutes to complete 8 questions.

The following questions were not written by College Board and, although they cover information outlined in the AP Physics 1 Course and Exam Description, the formatting on the exam may be different.

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1. A car is moving around a curve on an interstate highway at 55 mph. What force keeps the car, when turning on the curve, from skidding?

A. Kinetic friction

C. Normal force

D. Centripetal force

Answer: This one is tricky! Remember that when a tire rolls that the contact point between the tire and road is a "static" contact point. If the friction was kinetic, the tire would be sliding and the car would "skid" and not be able to go through the curve. Also watch out for "centripetal force"...always use the "true" force (friction, normal force, tension, etc.) to represent the inward force.

📄 Study AP Physics 1, Unit 3.1: Vector Fields

A. The riders' seatbelts are loose giving them a feeling of "weightlessness".

B. The riders are getting "pushed out" of their seats and therefore feel lighter at point "a".

C. The normal force for the riders is greater than the weight of the riders, mg.

Answer: If you draw a free-body diagram for the riders at "a", you would have a shorter arrow pointing up representing the normal force (what they THINK they weigh) and a longer arrow pointing down representing the weight (mg) of the riders. The resulting net force, the centripetal force, would point in toward the center of the circle pictured.

📄 Study AP Physics 1, Unit 3.7: Free-Body Diagrams for Objects in Uniform Circular Motion

B. The normal force for the riders is smaller in magnitude than their weight, mg.

C. The riders are getting "pushed down" in their seats and therefore feel heavier at point "b".

D. The riders' seatbelts are too tight giving them a feeling of "heaviness".

Answer: If you draw a free-body diagram for the riders at "b", you would have a longer arrow pointing up representing the normal force (what they THINK they weigh) and a shorter arrow pointing down representing the weight (mg) of the riders. The resulting net force, the centripetal force, would point in toward the center of the circle pictured.

📄 Study AP Physics 1, Unit 3.7: Free-Body Diagrams for Objects in Uniform Circular Motion

A. A

C. C

D. D or E

Answer: As something moves in a circle the string is pulling the object inwards due to the force of tension (the centripetal force). If the object is "let go" as when the string breaks, the object will travel in a straight line tangent to the circle where the force stops acting on the object.

📄 Study AP Physics 1, Unit 3.6: Centripetal Acceleration and Centripetal Force

A. The satellite because Jupiter is so much more massive

B. Jupiter because the satellite has such little mass

D. It depends on the distance between the two objects

Answer: Newton's 3rd law--the force between two objects is always equal and opposite. Therefore, Jupiter can only pull on the satellite with the same force that the satellite pulls back on Jupiter.

📄 Study AP Physics 1, Unit 3.8: Applications of Circular Motion and Gravitation

B. Outward, perpendicular to the road.

C. Tangent to the curve in the forward direction.

D. Tangent to the curve opposite the direction of the car's motion.

Answer: In order for the motorcycle to stay on the road, the force of friction (the centripetal force) has to be acting toward the center of the imaginary circle formed by the curve in the road. This means that the friction force must point at a 90 degree angle or perpendicular to the line of motion at that point.

📄 Study AP Physics 1, Unit 3.6: Centripetal Acceleration and Centripetal Force

A. 0 N

B. 5 N

C. 10 N

Answer: For this question you'll need to use the equation F = mv^2/r. The tricky part of the math is that you need to recognize the 2.0 m is the diameter and not the radius of the circle. F = (5*2^2)/1 = 20 N of force.

📄 Study AP Physics 1, Unit 3.1: Vector Fields

A. Centripetal force

B. Weight

C. Normal

Answer: In this case, it is the friction between the coin and the surface of the turntable that keeps the coin from sliding. The normal force is part of the force of friction, however, it is not the only force keeping the coin from sliding. Remember, centripetal force is a term used for the "center-seeking" force but the "true" force is used to explain why the coin stays on the turntable.

📄 Study AP Physics 1, Unit 3.7: Free-Body Diagrams for Objects in Uniform Circular Motion

A. The same force as the original

B. 2 times the original force

D. Cannot be determined without knowing the acceleration due to gravity

Answer: For this question you should use F = G(m1*m2)/r^2. Since the radius and G don't change, your proportionality is F ~ m1*m2. Since both masses double, Fnew ~ 2m*2m = 4*Foriginal.

📄 Study AP Physics 1, Unit 3.3: Gravitational and Electric Forces

B. 1/2 of the original force

C. 2 of the original force

D. 4 of the original force

Answer: For this question you should use F = G(m1*m2)/r^2. Since the masses and G don't change, your proportionality is F ~ 1/r^2. Since the radius doubles, Fnew ~ 1/2^2 = 1/4*Foriginal.

📄 Study AP Physics 1, Unit 3.3: Gravitational and Electric Forces

A. The acceleration is 1/4 of the original.

C. The acceleration is 2 times of the original.

D. The acceleration is 4 times of the original.

Answer: Knowing that ac = v^2/r and that the speed stays the same, the proportionality is ac ~ 1/r. If the radius is doubled, then ac(new) = 1/2 ac(original).

📄 Study AP Physics 1, Unit 3.8: Applications of Circular Motion and Gravitation

A. 1/4 of the original

B. 1/2 of the original

D. 4 times the original

Answer: Using the equation Fc = mv^2/r, if you double the mass and keep the speed and the radius the same, then the proportionality becomes Fc ~ m. Doubling the mass then makes the proportionality Fc(new) = 2*Fc(original).

📄 Study AP Physics 1, Unit 3.1 Vector Fields

A. Weight

B. Force due to gravity

C. Tension

Answer: At the top of the circle, both mg and the normal force act on the person. The normal force keeps the person in the seat since it is the contact force between the rider and the car.

📄 Study AP Physics 1, Unit 3.1: Vector Fields

B. Speed

C. Mass

D. Radius

Answer: Remember that velocity is a vector and changes as the direction changes! Speed, mass, and radius will not change for a particular circle.

📄 Study AP Physics 1, Unit 3.6: Centripetal Acceleration and Centripetal Force

A. Tension

C. Weight (mg)

D. Friction

Answer: Your "apparent weight", or the weight you "feel", is really your normal force, not your "actual weight". Your "actual weight" is determined by your mass and the acceleration due to gravity (mg).

📄 Study AP Physics 1, Unit 3.1: Vector Fields