AP Physics 1 : Fundamentals of Force and Newton's Laws

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

Example Question #1 : Newton's Third Law

A man shoots a rifle and the force of the shot results in recoil. The magnitude of the force on the rifle __________ the magnitude of force on the bullet, and the magnitude of acceleration of the rile __________ that of the bullet.

Possible Answers:

is less than . . . is greater than

equals . . . equals

is less than . . . is less than

is greater than . . . is less than

equals . . . is less than

Correct answer:

equals . . . is less than

Explanation:

Consistent with Newton's third law, which states that every force has an equal and opposite reaction, the force on the rifle is equal to the force on the bulet. However, the rifle has a larger mass, so the magnitude of its acceleration is less than that of the bullet.

Example Question #1 : Newton's Third Law

Two cars, one of mass 500kg and one of mass 250kg, collide head on. The car with more mass experiences a(n) __________ force and a(n) __________ acceleration with respect to the the smaller car. 

Possible Answers:

larger . . . smaller 

equal . . . larger 

larger . . . larger 

equal . . . smaller

Correct answer:

equal . . . smaller

Explanation:

The cars will experience the same force, due to Newtown's third law. Because the larger car experiences the same force and has a larger mass, by Newton's first law, it will have a smaller acceleration. 

Example Question #6 : Newton's Third Law

A book exerts a force of 2N downwards, into a chair that exerts a force of 5N dowwards to the floor it stands on. What is the force that the floor exerts upwards on the chair? 

Possible Answers:

Correct answer:

Explanation:

By Newton's third law, for every reaction there is an equal and opposite reaction. The floor must exert a 7N force upwards on the chair for the system to remain at rest. If it exerted less than that, the chair would be accelerating into the floor. This force, exerted by a surface, perpendicular to it, is called the normal force.

Example Question #7 : Newton's Third Law

A football player, feeling aggressive, is picking on a kid much smaller than himself. The football player asserts: "All of my hours in the weight room prove I can hit you harder than you hit me!" The smaller and more reserved gentleman replies: "Ya of course you can. I'm much weaker than you". Assuming by "hit" they mean "apply a force", are these two correct to think the football player can hit harder? Why?

Possible Answers:

No, Newton's third law says that the force two objects act on each other is always EQUAL in magnitude and opposite in direction.

No, Newton's first law says that the force two objects act on each other is always EQUAL in magnitude and opposite in direction.

Yes, Newton's second law says that the football player will cause the smaller gentleman to accelerate to a greater extent. Therefor he exterts a greater force.

Yes, Newton's second law says that football-playing-high-school bullies always hit harder.

Correct answer:

No, Newton's third law says that the force two objects act on each other is always EQUAL in magnitude and opposite in direction.

Explanation:

The relative mass of two interacting objects does not influence the magnitude of the force that the two objects exert on each other. Newton's third laws states the force must be equal in magnitude. If you're trying to reconcile how a football player is unable to "hit harder" than someone who does not lift weights, the answer lies in Newton's second law.

 

The football player weighs more and thus experiences a small acceleration. The smaller gentleman experiences a large acceleration due to his relatively small mass. This large acceleration is what we view when we see a large football player hit someone smaller. The football player hardly changes his motion while the smaller person will fly backwards. 

Example Question #1 : Newton's Third Law

Describe the phenomenon of a seat belt keeping someone restrained in their seat during a car crash. In other words, why did the person not leave their seat?

Possible Answers:

Newton's third law

Tension force law

Newton's first law

Newton's second law

All the laws describe why the person was not ejected from their seat

Correct answer:

Newton's third law

Explanation:

While all three laws come into play in a car crash only one is specifically responsible for keeping a person restrained in their seat. Just before the crash, the passenger is moving along at the same speed as the car. When the car collides into the other car and decelerates, the person's body continues to move forward. This is Newton's first law, or the law of inertia. But it is Newton's third law that keeps the person from being ejected from their seat. The force of the person's body moving forward was matched by that of the seatbelt. For every action there is an equal and opposite reaction. If the seat belt was unable to "match" the force of the person's body moving forward against it, it would have snapped and the person would have continued forward.

Example Question #9 : Newton's Third Law

A locomotive is pushing a train at a constant velocity of . How does the force exerted by the locomotive on the train relate to the force of the train on the locomotive?

Possible Answers:

They are in opposite directions and have different magnitudes

They are the same in magnitude and opposite in direction

They are in the same direction and have the same magnitude

They are the same in both magnitude and direction

None of these

Correct answer:

They are the same in magnitude and opposite in direction

Explanation:

This is Newtons's third law. Every action has an equal an opposite reaction. The reason the train is able to move at all is due to the force the locomotive puts on the rails, which enables it to accelerate the cars.

Example Question #81 : Fundamentals Of Force And Newton's Laws

Two objects apply forces to each other. The force on one of the blocks as a function of time in the x-direction is , where  and  are constants. What's the force as a function of time in the x-direction on the other block? Assume no other forces are present besides the forces the objects apply to each other.

Possible Answers:

Correct answer:

Explanation:

By Newton's third law, every action has an equal and opposite reaction. So the force has an equal and opposite force on the other block. Mathematically, this just means to negate the force.

Example Question #11 : Newton's Third Law

Two men are having a shoving contest, each man pushes the other at the exact same time. Ross pushes Jake with of force and Jake pushes Ross with of force. How much force is exerted on Jake?

Possible Answers:

Correct answer:

Explanation:

The correct answer is because of Newton's Third Law which states that for every action there is an equal and opposite reaction, so when Jake pushes Ross he also exerts the force he uses on himself. With this plus the force Ross exerts on Jake we arrive at our answer of

Example Question #1 : Force Diagrams

Consider the following system:

Slope_2

If the mass accelerates down the plane at a rate of  and the angle , what is the coefficient of kinetic friction between the mass and slope?

Possible Answers:

Correct answer:

Explanation:

Before we start using equations, we need to determine what forces are acting on the block in this system. The only relevant forces in this situation are gravity and friction. We are given the acceleration of the block, giving us the tools to find the net force.

Using Newton's second law, we can write:

The force of friction is subtracted because it is in the opposite direction of the movement of the block. Substituting in expressions for each variable, we get:

Canceling out mass and rearranging for the coefficient of kinetic friction, we get:

We have values for each variable, allowing us to solve:

Example Question #2 : Force Diagrams

Consider the following system:

Slope_2

This system is set on a different planet, which has a gravitational constant different from that on Earth. If the angle measures , the coefficient of kinetic friction is , and the block is accelerating at a rate of , what is the gravitational acceleration on this planet?

Possible Answers:

Correct answer:

Explanation:

There are two relevant forces acting on the block in this scenario: gravity and friction. We can use Newton's second law to solve this problem:

Substituting in expressions for each force, we get:

Eliminating mass and rearranging for , we get:

At this point, we can plug in values for each variable and solve:

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