AP Physics 1 - 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?

$g=10\frac{m}{s^2}$

$2\frac{m}{s^2}$ upwards

$10\frac{m}{s^2}$ downwards

$5\frac{m}{s^2}$ upwards

$2.5\frac{m}{s^2}$ 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:

$m=\frac{100N}{10\frac{m}{s^2}}=10kg$

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

$a=\frac{20N}{10kg}=2\frac{m}{s^2}$

Suppose that two people skydive out of a plane. Person A is , while person B is . After both people open their parachutes, they begin to slow down until they both reach a constant velocity. How does the net force between persons A and B differ?

The net force on person A and B are equal

The net force on person A is twice as great

The net force on person B is twice as great

The net force on person A is four times as great

The net force on person B is four times as great

Explanation

For this question, we're told that two skydivers of different masses are jumping out of an airplane. At some point, both skydivers slow down to the point at which they reach constant velocity.

To answer this, we have to realize how constant velocity affects the net force. Since the velocity for both people is constant, that means that no acceleration is occurring. Moreover, because either person is not accelerating, we see that the net force on each person is zero. Consequently, the net force for both person A and B is the same.

A box is sliding down a plane that has a inclination angle of $\theta = 10^{\circ}$ . Calculate the coefficient of kinetic friction $\mu _k$ 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:

$\sum F_{x'}=0=mg \sin \theta - F_f \Rightarrow mg \ sin \theta = \mu _k N$

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

$\sum F_{y'}= 0 = N - mg \cos \theta \Rightarrow N = mg \cos \theta$

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

$mg \ sin \theta = \mu _k mg \cos \theta\Rightarrow \mu _k = \tan \theta = \tan 10^{\circ}= 0.176$

A 1675 kilogram car is moving to the right at a constant velocity of $25\frac{m}{s}$ .

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.

A spaceship travels in the vacuum of space at a constant speed of $500\frac{m}{s}$ .

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 $500\frac{m}{s}$ indefinitely and the net force on the spaceship must be zero. This can also be shown mathematically:

$F=(1000kg)(0\frac{m}{s^2}) = 0N$

What net force is required to keep a 500 kg object moving with a constant velocity of $10 :\frac{m}{s}$ ?

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.

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

Dropping a box causes it to accelerate downwards.

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

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

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.

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.

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?

An object at rest tends to stay at rest

None of these

A larger/more massive object means a larger inertia

Acceleration is the time derivative of velocity

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.

A skydiver of mass has reached terminal velocity at $52 \frac{m}{s}$ . Estimate the force of wind resistance she is experiencing.

$F_{wind}=901.6N$

None of these

$F_{wind} = 1024.1N$

$F_{wind}=705.6N$

$F_{wind}=551.6N$

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.

$F_{net}=0$

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

$F_{net}=0=mg+F_{wind}$

Where is pointing down and thus negative

Plug in values:

$F_{net}=0=92*-9.8+F_{wind}$

Solve for $F_{wind}$

$F_{wind}=901.6N$

If an object is being accelerated, what must be true about the forces acting on it?

The sum of the forces $\sum \vec{F} eq \vec{0}$

There are no forces acting

There is only one force $\vec{F}$ acting

The forces acting are perpendicular to the motion of the object

Explanation

From the definition of Newton's first law, for an object to not be accelerated, $\sum \vec{F}=\vec{0}$ . Therefore, for an object to beaccelerated, $\sum \vec{F} eq \vec{0}$ .

Newton's First Law

Help Questions

Explanation

Explanation

Explanation

Explanation

Explanation

Explanation

Explanation

Explanation

Explanation

Explanation