Forces Unit Review

Forces Unit Review

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Multiple Choice

Consider the free body diagram below. What would the force of friction need to be for the object to be able to increase its speed?

• less than 24 N

• exactly 24 N

• more than 24N

• cannot say without knowing mass

Multiple Choice

A 4.0-kg object is moving across a friction-free surface with a constant velocity of 2 m/s. Which one of the following horizontal forces is necessary to maintain this state of motion?

• 0 N

• 0.5 N

• 2.0 N

• 8.0 N

Multiple Choice

Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose that you visit that place (just suppose) and throw a rock. The rock will

• gradually stop

• continue in motion in the same direction at constant speed.

Multiple Choice

If the forces acting upon an object are balanced, then the object

• must not be moving

• must be moving with a constant velocity

• must not be accelerating

• none of these

Multiple Select

Select all possible motions for this free body diagram.

• at rest - velocity is 0 m/s

• constant velocity to the right 3 m/s

• constant velocity up at 3 m/s

• accelerating at 3 m/s²

Multiple Choice

Select the FBD that matches the scenario. A rightward-moving car skids to a stop (with locked wheels).

Netwon's 1st Law - Objects with balanced forces will maintain their current velocity until acted on by an unbalanced force

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Multiple Choice

A melon has a mass of 2 kg. The force of gravity on the apple is..

• 10 N Down

• 2 N Down

• 20 N Down

F_g = m * 10m/s²

• F_g = 2 kg * 10 N/kg

• Fg = 20 N

• THIS IS ALSO ITS WEIGHT!!!!!!!!!!

Multiple Choice

A 12 N force is applied to a 3 kg object and a then to a 6 kg object. Which statement is true about their accelerations

• The 3 kg object will accelerate faster

• The 6 kg object will accelerate faster

• The objects will accelerate at the same rate

F=ma rearranges to F/m = a

• 12N/3kg = 4m/s²

• 12N/6kg = 2m/s²

Multiple Choice

You apply a 10N force to two boxes. Box A accelarates a rate of 5 m/s² and Box B accelerates at a rate of 2 m/s². Which statement is true.

• Box A has more mass

• Box B has more mass

F=ma rearranges to F/a = m

• Box A 10N / 5m/s² = 2 kg

• Box B10N / 2m/s² = 5 kg

Multiple Choice

While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?

• The force on the firefly

• The force on the bus

• The forces will be the same magnitude

Multiple Choice

Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ...

• greater than the acceleration of the bullet

• smaller than the acceleration of the bullet.

• the same size as the acceleration of the bullet.

Newton's 3rd Law

Forces come in Pairs

Force Pairs are between two objects

Force Pairs are equal and opposite

Multiple Choice

This force apposes motion

• Gravity Force

• Normal Force

• Friction Force

• Tension Force

Multiple Choice

This force prevents objects on surfaces from accelerating towards the center of the earth.

• Gravity Force

• Tension Force

• Normal Force

• Applied Force

Multiple Choice

Select the FBD that represents the forces an astronuat standing on the floor of the rocket ship will experience as the rocket ship takes off.

Multiple Choice

What is the NET force on the object described in this free body diagram?

• 20 N Left

• 120 N Left

• 20 Right

• 120 N Right

Multiple Choice

What is the Fnet on this object?

• 2 N Up

• 4 N Up

• 6 N Up

• 8 N Up

Fnet = sum of all forces

• Vertical = +8N - 6N = 4N

• Horizontal = +12N -12N = 0N

• Fnet = 2N Up

Multiple Choice

What is the wieght of a 12.4 kg object on the moon where the acceleration due to gravity is 1.6 m/s² ?

• 1.6 N

• 12.4 N

• 7.75 N

• 19.84 N

F = m * a

• F = 12.4 Kg x 1.6 m/s²

• F = 19.84 N

Multiple Choice

If an object on the Moon, where the acceleration due to gravity is 1.6 m/s² , had a Weight of 150 N, what would its mass be?

• 1.6 Kg

• 93.75 Kg

• 150 Kg

• 250 Kg

F = m * A

• rearrange F/a= m

• 150N/1.6m s^2 = m

• 93.75 Kg = m

Multiple Choice

A dog pulls a bone that weighs 19.6 N to the right with a 20 N force. There is an friction force of 10 N. What is the acceleration of the bone?

• 1 m/s² to the right

• 5 m/s² to the right

• 10 m/s² to the right

• 15 m/s² to the right

F=ma

F net - Vertical = 0 N;
Horizontal 10 N Right

m=F/g = -19.6/-9.8 = 2kg

a = Fnet/m = 10N/2kg = 5 m/s^2

Multiple Choice

A 500 kg piano is accelerated accross room at a rate of 0.25m/s2 to make room for a christmas tree. There is a friction force of 50 N.  What is the Fapp? Draw a FBD of the object

• 12.5 N

• 75 N

• 125 N

• 175 N

F = ma

• Fnet = 500 Kg & 0.25 m/s²

• Fnet = 125 N

• Also...

• Fnet = F_app - F_friction

• 125N = F_app - 50 N

• 125N + 50 N = F_app

• 175N = F_app

Multiple Choice

A student sets off a 0.5 kg play rocket with an acceleration of 3 m/s² . What was the force of thrust needed to launch the rocket? Also draw a fully labeled FBD.

• 1.5 N Up

• 3.4 N Up

• 6.4 N Up

• 7.35 N Up

F = ma

• Fnet = m * a = 0.5 kg * 3 m/s²

• Fnet = 1.5 N

• also....

• Fnet = F_thrust - F_grav

• 1.5N = F_thrust - 4.9 N

• 1.5N + 4.9N = F_thrust

• 6.4 N = F_thrust