# AP Physics C: Mechanics : Forces

## Example Questions

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### Example Question #1 : Friction And Normal Force

Weightlessness is experienced when the normal force equals __________.

the mass of the object

the force due to gravity

a negative value

zero

zero

Explanation:

"Weightlessness" is analogous to free-fall (neglecting air resistance), during which the only force on an object is the force of gravity. When normal force becomes zero, the object loses physical contact with the surface.

When normal force is equal to the force of gravity, the object is in equilibrium and the net vertical force is zero. This results in zero acceleration, but does not result in "weightlessness." Mass is not a force, and negative force values merely indicate relative direction (assuming correct calculations)

### Example Question #71 : Ap Physics C

A book of mass 1kg is held to a vertical wall by a person's hand applying a 20N force directly toward the wall. The wall has a static friction coefficient of 0.3 and a kinetic friction coefficient of 0.2. With the book held at rest, what the is frictional force keeping the book from sliding down the wall?

Explanation:

Frictional force is calculated using the normal force and the friction coefficient. Since the book is at rest (not in motion), we use the coefficient of static friction.

The normal force will be equal and opposite the force pushing directly into the surface; in this case, that means the normal force will be 20N.

Solve for the force of friction:

Explanation:

Explanation:

Explanation:

Explanation:

Explanation:

### Example Question #1 : Friction

truck traveling at  suddenly brakes and skids to a halt. If the coefficient of kinetic friction between the tires and the road is , how far does the truck skid before stopping?

Explanation:

Relevant equations:

Determine the truck's change in kinetic energy. Note that the final kinetic energy will be zero because it has no final velocity.

Using the work-energy theorem, set change in kinetic energy equal to the work done by friction.

Substitute the equation for force of friction.

Use the given values for coefficient of friction, mass, and acceleration of gravity to solve for the distance.

### Example Question #1 : Friction

A force of 40N is applied on a 14kg box to move it to the right. If the coefficient of friction between the box and the floor is 0.2, what is the acceleration of the box?

Explanation:

The two horizontal forces acting on the box are the applied force of 40N to the right, and the friction force ()to the left. So the net horizontal force is written as the equation below.

is the normal force. Solving for acceleration, a, we get the equation below.

We know the following from the question.

Thus, the acceleration is

### Example Question #1 : Forces

crate is pulled across a horizontal floor at a constant velocity of . The coefficient of kinetic friction between the crate and the floor is . If the rope pulling the crate is angled at  above the horizontal, what force must be applied to maintain this constant velocity?

Explanation:

Relevant equations:

Write net force equations for horizontal and vertical directions:

Horizontal:

Vertical:

Since the crate is not accelerating,  in each direction, implying that .

Horizontal:

Vertical:

Express the horizontal and vertical components of the pulling force in terms of the total pulling force, .

Horizontal:

Vertical:

Replace force of friction with its expression in terms of the normal force.

Horizontal:

Rearrange to isolate the normal force.

Horizontal:

Substitute this term for the normal force in the net vertical force equation.

Vertical:

Solve to isolate the pulling force.

Use the given values for the mass of the box, coefficient of friction, and acceleration of gravity to solve for the pulling force.

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