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

## Example Questions

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### Example Question #1 : Newton's First Law

What net force is required to keep a 500 kg object moving with a constant velocity of ?

Not enough information

Explanation:

Newton's first law states that an object in motion tends to stay in motion unless if acted upon by a net force. This means that if friction is not being accounted for, there is no net force required to keep an object moving if it's in motion. A net force is only required to change an object's motion. The 500 kg object is moving at a constant velocity; therefore, there is no net force (0 Newtons) acting on the object.

### Example Question #2 : Newton's First Law

All of the following statements are true. Which of them is NOT explained by Newton's first law of motion?

A book lying on a desk doesn't start moving without an external force.

A baseball thrown in space will keep moving in the same direction until it hits something.

Dropping a box causes it to accelerate downwards.

If I'm riding a skateboard, it doesn't spontaneously reverse directions.

A ball rolling straight down a tilted plane hill won't veer left or right unless something pushes it.

Dropping a box causes it to accelerate downwards.

Explanation:

Newton's first law of motion refers to objects with no external forces acting on them. Objects with no external forces will maintain the same velocity, meaning that

(1) if they are not moving, they will continue not moving

(2) if they are moving, they will keep moving with the same speed and direction

The answer "Dropping a box causes it to accelerate downwards" refers to a box with a force acting upon it—the force of gravity. Also, the box is accelerating, unlike objects referred to by Newton's first law of motion, which have constant velocities.

### Example Question #2 : Newton's First Law

spaceship travels in the vacuum of space at a constant speed of

Ignoring any gravitational forces, what is the net force on the spaceship?

Explanation:

In a vacuum, there is no friction due to air resistance. Newton's first law states that an object in motion stays in motion unless acted upon by a net force. Thus the spaceship will travel at the constant speed (zero acceleration) of  indefinitely and the net force on the spaceship must be zero. This can also be shown mathematically:

### Example Question #3 : Newton's First Law

You are traveling on an airplane at constant speed of 650mph. Your friend is traveling in his car at a constant speed of 60mph. Who experiences a larger acceleration?

Neither you nor your friend

Cannot be determined; we must know the force due to friction

You

Neither you nor your friend

Explanation:

Since both you and your friend are traveling at a constant speed, the acceleration of you and your friend is zero. Thus, neither you nor your friend experiences any acceleration. This can be shown mathematically using the equation for acceleration:

Since there is no change in velocity over time acceleration is zero. Also note that when acceleration is zero, so is the net force.

### Example Question #2 : Newton's First Law

You are sitting in a car, at rest, when another car rear ends your vehicle. Why do you and the passengers experience a whiplash, in terms of Newton's laws of motion?

A larger/more massive object means a larger inertia

An object at rest tends to stay at rest

None of these

Acceleration is the time derivative of velocity

An object at rest tends to stay at rest

Explanation:

Because an object at rest tends to stay at rest, when your car is hit your body/neck will 'want' to stay where it was. This will cause your body and neck to 'whip' as it will take time for it to speed up from being hit.

### Example Question #4 : Newton's First Law

A passenger in an elevator has a mass that exerts a force of 100N downwards. He experiences a normal force upwards from the elevator's floor of 120N. What direction is he accelerating in, if at all, and at what rate?

upwards

downwards

upwards

upwards

upwards

Explanation:

The acceleration of the person in the elevator is determined by the net forces and his/her mass. The net force is calculated to be 20N upwards. To find the mass of the passenger, use the following formula:

Then, to find the net acceleration, use Newton's second law.

### Example Question #5 : Newton's First Law

A 1675 kilogram car is moving to the right at a constant velocity of .

What is the net force exerted on the car?

Explanation:

Recall Newton's first law of motion: an object will remain in its state of uniform motion unless acted upon by an external force. The car's motion is described as having a constant velocity which is a uniform state so there are no external forces.

### Example Question #1 : Newton's First Law

A box is sliding down a plane that has a inclination angle of . Calculate the coefficient of kinetic friction  if the box is moving at a constant velocity .

Explanation:

If the box is moving at a constant velocity, we know that the sum of the forces acting up and down the inclined plane must add to zero. If the only forces acting are gravity and friction, we can show:

We can gain information about the normal force  by looking at the forces acting perpendicular to the plane, shown by:

Putting all of this together lets us write the following expression for the forces in acting along the plane, and finally, our answer:

### Example Question #7 : Newton's First Law

A spacecraft is deep in space and motionless. Then, the spacecraft separates into two sections going in opposite directions. The first has a mass of and is moving with a magnitude of velocity of . The second has a mass of . Determine the magnitude of velocity of the second section.

Explanation:

Use conservation of momentum:

Plug in values.

Solve for :

### Example Question #8 : Newton's First Law

A skydiver of mass  has reached terminal velocity at . Estimate the force of wind resistance she is experiencing.

None of these

Explanation:

If the diver has reached terminal velocity, her acceleration is .

Thus, according to Newton's second law:

Her net force is equal to zero.

The only forces acting on her are gravity and wind resistance, which must add up to zero.

Where is pointing down and thus negative

Plug in values:

Solve for

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