# AP Physics 1 : Other Force Fundamentals

## Example Questions

### Example Question #1 : Linear Motion

Which answer choice below includes only scalar quantities?

Velocity, displacement, force

Distance, speed, time

Force, time, velocity

Acceleration, energy, displacement

Displacement, time, acceleration

Distance, speed, time

Explanation:

Scalar quantities are those that can be described with magnitude only, as opposed to vectors, which include both magnitude and direction components. Distance, speed, and time are all scalars. Displacement is not a scalar, as it involves both the distance and the direction moved from a starting point. Velocity also includes a direction component, and is therefore a vector quantity.

### Example Question #1 : Other Force Fundamentals

A person who weighs 700 N is standing on a scale that is on the floor of an elevator lift. If the elevator accelerates downward at , what will the scale read?

Explanation:

According to Newton's second law,

There are two forces acting on the person standing on the scale in the elevator. Those two forces are the person's weight due to gravity (which acts in a downward direction), and the normal force from standing on the scale (which acts in an upward direction).  So, the net force equation can be written as:

represents the person's weight, which was given in the problem to be 700 N.   is the normal force due to standing on the scale; this is what we are trying to find, because it is what the scale would read. We subtract the two forces because they point in opposite directions, and so one must be positive and the other negative.

We also know the acceleration, , is .

We were not given the mass of the person, but we can find it because we know the person's weight is 700 N. The equation that relates a person's mass to their weight near the surface of the Earth is:

Now we can solve for the normal force by using the mass we found in the original equation above:

### Example Question #1 : Other Force Fundamentals

What units make up a Newton?

Explanation:

The answer is exemplified by the centripetal force equation:

.

When we combine the units of each of the equation's components, we arrive at

which, when simplified, becomes

### Example Question #3 : Other Force Fundamentals

Two toy cars are moving on a frictionless track. Car 1 is moving at  when it collides into Car 2, which was at rest. After the collision, Car 2 moves at  and Car 1 moves at . If Car 1 weighs 0.05 kg, how much does Car 2 weigh?

Explanation:

First, lets identify the given information:

Since this is a collision problem, we can apply the law of conservation of momentum:

Because Car 2 is initially at rest, this equation can be simplified:

The equation can then be rearranged to solve for the mass of Car 2:

By plugging in the known values, we can determine the mass of Car 2:

### Example Question #1 : Other Force Fundamentals

Consider a cube with a mass of  and sides of . Calculate the pressure, , on the floor from such a cube.

Explanation:

To calculate the pressure  on the floor, we need to know the force  acting on the floor, as well as the area  over which it acts.

The force acting is simply the weight of the cube,

The area is the area of one side of the cube as it rests on the floor,

Therefore, the pressure  is given as:

Plug in known values and solve.

### Example Question #1 : Other Force Fundamentals

During a mission to the Moon, astronauts were traveling towards the moon at , however, for most of the flight to and from the Moon, they were not slammed into their seats, and could move freely about the aircraft, why is that?

None of these

Because there is no gravity in space

Because they were being pulled towards the Moon

The astronauts had very strong muscles and could push themselves up

Because they were not accelerating

Because they were not accelerating

Explanation:

Once the spacecraft had reached a constant speed, so had it's occupants. Thus, the astronauts were also moving at and they were experiencing no forces from their seats.

Thus, the astronauts were experiencing zero net force.

### Example Question #6 : Other Force Fundamentals

Three forces act on an object that is in equilibrium and suspended in air.  is  pulling on the object  from the positive horizontal axis.  is  pulling on the object  from the negative horizontal axis.  is acting on the object straight down on the negative vertical axis. What is the magnitude of

Explanation:

and  cancel each other out in the horizontal direction due to the fact that they are equal and opposite about the vertical axis. Now that the resultant force in the x-direction is zero, we must find what  must be in order to zero out the other two forces' y-components, which are:

These forces are pointing in the positive y-direction (pulling the object up). Therefore, in order to have a resultant force of zero in the vertical direction, there must be a downward force of

Since we are asked for the magnitude, we are not particularly concerned with the sign of this value.

### Example Question #2 : Other Force Fundamentals

A cowboy is lassoing a cow at the rodeo. Identify the force that is causing the object to travel in a circular path.

Tension

Electrical

Normal

Spring

Gravity

Tension

Explanation:

In these conceptual problems, it is always best to draw an accurate force diagram. In string problems, there is always a tension force, and here, it is causing the circular motion. The tension force causes the lasso to swing in a circle and pulls it toward the center of the circle.

### Example Question #8 : Other Force Fundamentals

Which of the following is not one of Newton's fundamental laws?

All of these correctly represent Newton's laws

For every action, there will always be an opposite reaction equal in magnitude

An object in motion will remain in that state of motion unless an outside force acts on that object

The acceleration experienced by an object is directly proportional to the force applied to that object, and inversely proportional to the mass of that object

In order for an object to be in motion, a force needs to act on it

In order for an object to be in motion, a force needs to act on it

Explanation:

To answer this question, we'll need to be familiar with Isaac Newton's fundamental laws of physics, which forms the basis of classical mechanics.

Newton's first law states that an object in motion will remain in that state of motion indefinitely, unless an outside force acts on this object.

Newton's second law can be stated as follows:

This expression tells us that the acceleration experienced by an object is directly proportional to the applied force and inversely proportional to the object's mass.

Newton's third law states that for every action, there is always an equal and opposite reaction. Another way of saying this is that if one object were to exert a force on a second object, then the second object would exert the same force against the first object.

One of the answer choices states the following

"In order for an object to be in motion, a force needs to act on it."

While this may seem intuitive based on our everyday experience, and it may also show some resemblance to Newton's first law, this statement is actually incorrect. Force is not responsible for motion - force is responsible for changes in motion.

### Example Question #1 : Other Force Fundamentals

You and your friend are pushing on a box with a force parallel to the ground of 40 N. The box weighs 5 kg, and the coefficient of kinetic friction between the box and the ground is 0.75. What is the net force on the box in the horizontal direction?