High School Physics : Calculating Force

Example Questions

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Example Question #1 : Calculating Force

Jerry pushes a  box with  of force. What is the resultant acceleration?

Explanation:

Newton's second law states that .

In this problem,  and .

Plug these into the equation to solve for acceleration.

Example Question #2 : Calculating Force

ball rolls forward with a net acceleration of . What is the net force on the ball?

Explanation:

Newton's second law states that .

Plug in the values given to us and solve for the force.

Example Question #1 : Calculating Force

Louisa rolls a ball with  of force. She observes that it has a constant linear acceleration of . What is the mass of the ball?

Explanation:

Newton's second law states that .

Plug in the given values to solve for the mass.

Example Question #1 : Calculating Force

Derek pushes a  crate along a rough surface with  of force. He observes that it is only accelerating at a rate of . What must the value of the force of friction be?

(Assume the only two forces acting on the object are friction and Derek).

Explanation:

Newton's second law states that .

If Derek is pushing with  of force, then we should be able to solve for the acceleration of the crate.

Derek observes that the crate is acceleration at a rate of , rather than the expected . An outside force is acting upon it to slow the acceleration.

The equation for the net force on the object is: . We also know, from Newton's second law, that , where the resultant force and acceleration are the values actually observed.

Plug in the information we've been given so far to find the force of friction.

Subtract  from both sides to find the force of friction.

Friction will be negative because it acts in the direction opposite to the force of Derek.

Example Question #2 : Calculating Force

A hammer exerts a  force on a nail. What is the force the nail exerts on the hammer?

Explanation:

Newton's third law states that when one object exerts a force on a second object, the second object exerts a force equal in size, but opposite in direction to the first. That means that the force of the hammer on the nail and the nail on the hammer will be equal in size, but opposite in direction.

Since the hammer exerts  of force on the nail, the nail must exert of force on the hammer.

Example Question #6 : Calculating Force

Michael pushes a  box with  of force to the left. Annie pushes the same box with  of force to the right. What is the net force on the box?

Explanation:

For the net force, we add up all the forces: .

Since force is a vector, the direction of the action matters. We will make leftward motion negative and rightward motion positive. Michael is pushing with  to the left, making his force equal to . Annie was pushing with  to the right, so her force will remain .

We can find the net force by adding the individual force together.

Example Question #2 : Calculating Force

Franklin lifts a  weight vertically. If he lifts it so that way the velocity of the weight is constant, how much force is he using?

Explanation:

If the object has a constant velocity, that means that the net acceleration must be zero.

In conjunction with Newton's second law, we can see that the net force is also zero. If there is no net acceleration, then there is no net force.

Since Franklin is lifting the weight vertically, that means there will be two force acting upon the weight: his lifting force and gravity. The net force will be equal to the sum of the forces acting on the weight.

Since we just proved that the net force will equal zero, we can say .

We know the mass of the weight and we know the acceleration, so we can solve for the lifting force.

Example Question #1 : Calculating Force

box starts at rest and reaches a velocity of  after traveling a distance . What was the force on the box?

Explanation:

The formula for force is .

We are given the mass, but we will need to calculate the acceleration to use in the formula.

We know the initial velocity (zero because the box starts from rest), final velocity, and distance traveled. Using these values, we can find the acceleration using the formula .

Plug in our given values and solve for acceleration.

Divide both sides by .

Now we know both the acceleration and the mass, allowing us to solve for the force.

Example Question #9 : Calculating Force

An airplane has a mass of . What is the minimum necessary lift force to get the airplane off the ground?

Explanation:

In this problem there will be two forces acting upon the airplane: the weight of the plane (force of gravity) and the lifting force. Since we are looking for the minimum force to lift the plane, we can set the two forces equal to each other: .

We can calculate the gravitational force using the mass.

Returning to the original equation, we see that the lifting force must be .

Example Question #1 : Calculating Force

How much force is required to acclerate a  crate at ?