High School Physics : High School Physics

Study concepts, example questions & explanations for High School Physics

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Example Questions

Example Question #8 : Identifying Forces And Systems

 A ball is throwing and is moving upward through the air.  Which free body diagram represents the force on the ball?  Neglect the effect of air resistance

4

 

Possible Answers:

Diagram A

Diagram C

Diagram B

Diagram D

Correct answer:

Diagram A

Explanation:

Once the ball leaves your hand, there is no longer a contact force applied on the ball in the upward direction.  There are no horizontal forces at work either.  The only force acting on the ball is gravity.  Gravity points in the downward direction.  Therefore Diagram A is the correct answer.

Example Question #9 : Identifying Forces And Systems

An object moves forward with a constant velocity. What additional information do we need to know to determine the force acting upon the object? 

Possible Answers:

The mass of the object

The distance the object travels

 

The force is 

The time the object is in motion

The velocity of the object

Correct answer:

The force is 

Explanation:

Force is given by the product of mass and acceleration. If an object has a constant velocity, then it has no acceleration.

 

 

If an object has no acceleration, then it must also have no net force.

 

 

 

 

No additional information is needed to solve this question.

 

 

 

Example Question #61 : Specific Forces

What is the acceleration of two falling skydivers (total mass =  including parachute) when the upward force of air resistance is equal to one fourth of their weight?

Possible Answers:

Correct answer:

Explanation:

There are two forces acting on the falling skydivers.  The first is the force of gravity (their weight) which we will denote as .  The second is the force of air resistance pushing up against them which we will denote as .

 

Newton’s 2nd Law tells us that the net force acting on these two objects is equal to their mass times their acceleration.

 

 

The net force is the sum of both forces acting together.

 

 

We know that the air resistance acts opposite to gravity and is equal to ¼ their total weight.

 

 

Substitute this value into our net force equation.

 

 

This simplifies to

 

 

The force of gravity is calculated by multiplying the mass by the acceleration due to gravity.

 





 

Substitute in our known variables and solve 

 


Example Question #1 : Gravitational Field

Sally is to walk across a “high wire” that has been strung horizontally between two buildings that are  apart.  The sag (dip) in the rope when she stands at the midpoint is .  If her mass is , what is the tension in the rope at this point?

 

Possible Answers:

Correct answer:

Explanation:

The first thing is to identify the forces involved in this situation.  There is the force of gravity (or her weight) which is pulling down on the rope.  We can calculate this by

 





 

The other forces are the force of Tension on each side of the wire as she stands in the midpoint.  These two Tension forces are what hold up Sally and keep her from falling.  However, these two Tensions forces are at an  angle below the horizontal.  This means that we need to analyze the components of the Tension force.  The -components of each Tension force are equal in magnitude and opposite in direction as this is what keeps the rope connected to both buildings.  The -components of each Tension force are equal in magnitude and in the same direction as they both are keeping Sally up.  So we can sum up the forces acting in the -direction as:

 

 

Which can be simplified to

 

 

Since Sally is not accelerating, the forces are balanced and the net force must equal .

 

 

Earlier we calculated the force of gravity so we can substitute this in to find the Tension in the  Direction.

 

 

 

 

This is the Tension in the -direction.  However, the problem is asking for the overall Tension in the wire.  At this point, we must use trigonometric functions to determine the hypotenuse (the overall Tension) in the wire.  Since the -component of the Tension is the opposite side of the triangle from the  angle, we can use cosine to find our hypotenuse.

 

 

 

 


 

Example Question #62 : Specific Forces

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If the mass of the object is  and , what is the value of ?  Assume 

 

Possible Answers:

Correct answer:

Explanation:

 will be the weight of the object. Weight is a very specific force: it is the mass times gravity. As it turns out, the angle is irrelevant in finding weight.

 

Using Newton's second law and the given values for mass and gravity, we can solve for .

 

 

 

 

Note that the weight is negative, because it is acting in the downward direction.

 

Example Question #3 : Gravitational Field

ball falls off a cliff. What is the force of gravity on the ball?  Assume 

Possible Answers:

We need to know the time the ball is in the air in order to solve

We need to know the height of the cliff in order to solve

Correct answer:

Explanation:

Newton's second law states:

 

 

In this case the acceleration will be the constant acceleration due to gravity on Earth.

 

 

Use the acceleration of gravity and the mass of the ball to solve for the force on the ball.

 

 

 

The answer is negative because the force is directed downward. Since gravity is always acting downward, a force due to gravity will always be negative.

 

 

 

Example Question #121 : Forces

A woman stands on the edge of a cliff and drops two rocks, one of mass  and one of , from the same height. Which one experiences the greater acceleration?

Possible Answers:

The rock with mass 

 The rock with mass 

 We need to know the density of the rocks in order to solve

We need to know the height of the cliff in order to solve

They experience the same acceleration

Correct answer:

They experience the same acceleration

Explanation:

Even though the rocks have different masses, the acceleration on both will be , the acceleration due to gravity. We can look at Newton's second law to see the force experienced by the rocks:

 

 

When objects are in free-fall, the acceleration will be equal to the acceleration from gravity, regardless of the mass of the object.


 

Example Question #1 : Gravitational Field

A person stands on a scale in an elevator.  His apparent weight will be the greatest when the elevator

Possible Answers:

 Is moving upward at constant velocity

Standing still

 Is accelerating downward

 Is accelerating upward

 Is moving downward at constant velocity

Correct answer:

 Is accelerating upward

Explanation:

Consider a person standing on a scale in an elevator that is not moving.  The person is exerting a downward force onto the scale equal to their weight force.  The scale exerts a force upward that is equal to the downward force of gravity.  This upward force is the reading on the scale.

 

Now consider a person standing on a scale in an elevator that is moving at a constant velocity.  The person is exerting a downward force onto the scale equal to the force of gravity.  The scale exerts a force upward.  Because the elevator is moving at a constant velocity, these forces are balanced, and the scale would read the same as if you were not moving.

 

Now consider a person standing in an elevator that is accelerating.  There are still two forces, the downward force of gravity, and the upward normal force.  Since the elevator is accelerating, these forces are unbalanced in the direction of the motion.  If the elevator is accelerating down, gravity is larger than the normal force.  If the elevator is accelerating up, the normal force is greater than gravity.

 

Since the scale reads the magnitude of the normal force, the time when the normal force is the greatest is when the elevator is accelerating in an upward direction.

 

Example Question #1 : Gravitational Field

weight and a  weight are dropped simultaneously from the same height.  Ignore air resistance.  Compare their accelerations.

Possible Answers:

 The  weight accelerates faster because it has more inertia

The  weight accelerates faster because it is heavier

 They both accelerate at the same rate because they have the same weight to mass ratio.

 The  weight accelerates faster because it has a smaller mass

Correct answer:

 They both accelerate at the same rate because they have the same weight to mass ratio.

Explanation:

 

 

Example Question #2 : Gravitational Field

Jerry wants to lift a  ball with exactly enough force so that its upward velocity is constant. How much force should he use?  Assume 

Possible Answers:

Correct answer:

Explanation:

If the velocity on an object is constant, that means it has no acceleration. If it has no acceleration, that means that the net force on the object is equal to zero. We can see this conclusion by using Newton's second law.

 

 

 

 

Another way to think of  is the sum of all the forces. Since the only two forces acting upon the ball are gravity and Jerry's lifting force, we can see: .

 

Since the net force is zero, the magnitude of Jerry's force must equal the magnitude of the force of gravity, but in the opposite direction.

 

 

This means that once we find , then Jerry's lifting force will be the same magnitude but in the opposite direction. Use Newton's second law to find the force of gravity.

 

 

 

 

This means that Jerry's lifting force will be .

 

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