# High School Physics : Forces

## Example Questions

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### Example Question #1 : Understanding Force

Two dogs, one  and one , pull on a bone. They each pull with the same force, but in opposite directions. What is the acceleration of the bone?

The bone will fly into the air

The bone will accelerate towards the  dog

The bone will have positive, then negative acceleration

The bone will accelerate towards the  dog

The bone will have zero acceleration

The bone will have zero acceleration

Explanation:

For this problem, we are looking at the net force on the bone. Since both dogs are pulling with the same force, we can say the magnitude of the force of dog 1 equals the magnitude of the force of dog 2, but in opposite directions. Mathematically, .

To find the net force on the bone, we add the individual forces together.

We can substitute our forces into the equation for net force.

The net force is zero. Each dog pulls with equal force in opposite directions, allowing the total force to cancel out.

According to Newton's second law, . If the net force is zero, then the acceleration of the bone must also be zero.

### Example Question #2 : Understanding Force

A man is painting a house. He notices that there is a small drop of paint that is remaining perfectly still on the vertical wall. What conclusion can he draw about the paint?

We need to know the mass of the drop to draw any conclusions

We need to know the density of the drop to draw any conclusions

The force of friction on the drop is equal to the force of gravity on it

The paint is water based

The drop of paint is very close to the ground, thus giving it a low potential energy

The force of friction on the drop is equal to the force of gravity on it

Explanation:

If the drop is at rest, then that means that the net forces acting upon it are equal to zero. There are two forces acting on the drop: the force due to gravity and the frictional force of the paint on the wall.

Mathematically, we can set up an equation for the net force:

The forces of friction and gravity are going to be equal and opposite, causing the drop to remain still on the wall.

### Example Question #1 : Forces

How is it possible that two forces of equal magnitude can result in zero net force?

The forces are in opposite directions

The forces are parallel to one another

The forces are acting on different objects

The forces are perpendicular to one another

The forces are generated in a circle

The forces are in opposite directions

Explanation:

If two forces act on a single object, then the net force on the object is equal to the sum of the forces acting on it.

Forces are vector quantities, however. This means that all forces have a magnitude and a direction of action. When adding forces, we must take their directions into account. Directions are broken into horizontal and vertical portions for vector addition, and can be positive or negative based on the direction along the axis.

For a net force to be zero, one force must be positive and the other must be negative along a given axis. If the forces are of equal magnitude, then they must be acting in exactly opposite directions in order to cancel each other.

### Example Question #1 : Introduction To Forces

Which of the following is not a type of force?

Normal force

Kinetic force

Buoyant force

Friction force

Drag (Air resistance)

Kinetic force

Explanation:

Kinetic force is not a technically correct property. Kinetic energy can be used to generate force, but is not a force in itself. Forces require non-zero acceleration, meaning that they only exist when there is a changein velocity. A change in kinetic energy can indicate a change in velocity, leading to a non-zero force value.

Normal force is the force of a surface upon an object, frequently countering gravity. Friction force is the resistance between an object and a surface, and acts opposite the direction of the object's motion. Buoyant force is the upward force of a fluid on a submerged object. Drag, or air resistance, is a special form of friction force for an object moving through a fluid.

### Example Question #1 : Understanding The Relationship Between Force And Acceleration

Two children standing on a frictionless surface push off of each other with  of force. If one child has twice the mass of the other child, what is the ratio of the lighter child to the heavier child?

Explanation:

First, realize that the force that the lighter child exerts on heavier child is equal and opposite to the force the heavier child exerts on the lighter child, as per Newton's third law.

Using Newton's second law, we can re-write this equation.

The question tells us that , making  the heavier child and  the lighter child. We can use this in our equation as well.

We are looking for the ratio of  to , so we need to rearrange the equation.

First, the masses cancel out.

Then, divide both sides by .

The ratio of   to is .

### Example Question #2 : Understanding The Relationship Between Force And Acceleration

crate slides along the floor with a constant velocity. What is the net force on the crate?

Explanation:

The relationship between force and acceleration is .

Since the crate has a constant velocity, it has no acceleration.

If there is zero acceleration, that means there is no net force on the object, or .

### Example Question #3 : Understanding The Relationship Between Force And Acceleration

crate slides along a frictionless surface. If it maintains a constant velocity of , what is the net force on the object?

Explanation:

Newton's second law states that . We know the mass, but we need to calculate the acceleration.

Acceleration is the change in velocity per unit time.

Since the velocity does not change from one moment to the next, then there must be no net acceleration on the object.

Returning to Newton's second law, we can see that if there is no acceleration, then there is no net force.

### Example Question #1 : Understanding The Relationship Between Force And Acceleration

The same force is applied to two different objects. One object has a mass of , and the other has a mass of . Which one has the greater acceleration?

The accelerations are equal

We need to know the value of the force applied to solve

We need to know the value of  to solve

Explanation:

The equation for a force is:

We can write this equation in terms of each object:

We know that the force applied to each object will be equal, so we can set these equations equal to each other.

We know that the second object is twice the mass of the first.

We can cancel out the mass from each side, leaving a relationship between the two accelerations.

The acceleration on the first mass is twice the acceleration on the second; thus, the acceleration of the lighter mass is greater.

### Example Question #5 : Understanding The Relationship Between Force And Acceleration

Lance pushes a crate of mass  with  newtons of force. What is the resultant acceleration?

Explanation:

The formula for force is Newton's second law:

We are told in the question to use for the mass and for the force.

Now we can isolate the acceleration.

This also makes sense from a units perspective. Units for force are Newtons, which can be written as:

In our equation, we can see that Newtons are divided by mass:

This would result in the units for acceleration.

### Example Question #1 : Understanding The Relationship Between Force And Acceleration

A ball begins to roll with a velocity of . If no outside forces act upon it, what will be its velocity in ?

Explanation:

If there are no forces acting upon the object, then there is no acceleration. If there is no acceleration, then the object will move with a constant velocity.

Mathematically, we can look at Newton's second law and the formula for acceleration.

We know that the force is zero.

Since we know that the mass cannot be zero, the acceleration must be zero.

We can now use the formula for acceleration to see the effects on velocity.

We know that the acceleration is zero and that the time is ten seconds.

In order for this to be true, the initial and final velocities must be equal.

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