High School Physics : Newton's Laws

Study concepts, example questions & explanations for High School Physics

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Example Questions

Example Question #1 : Understanding Newton's Third Law

Which of Newton's laws explains why it is easy for you to lift a 1L jug of milk from the fridge, but impossible to lift a 1000L jug?

Possible Answers:

More than one of these

Newton's first law

Newton's second law

Newton's third law

None of these

Correct answer:

More than one of these

Explanation:

The answer is more than one. The two laws that come into play are Newton's first and second laws. Newton's first law is best known as the law of inertia. It states that an object in motion will stay in motion and an object at rest will stay at rest unless acted on by an outside force. Newton's second law relates the acceleration of an object to the mass and the forces acting on it with the equation . An object won't move or stop moving unless the forces acting on it are imbalanced. In the case of the milk jug, it remained at rest until an outside force, your hand, acted upon it, demonstrating Newton's first law. The second law is applicable because the amount of acceleration of the two milk jugs is inversely related to their mass. It takes a much stronger force to move the 1000L milk jug than it does the 1L milk jug. The acceleration is also much smaller for the two objects when the same force is applied because one weighs so much more than the other. You can visualize this by rearranging the second law equation:  . It is also beneficial to think in terms of two objects. Which is will accelerate more when you try and move it with the same force, a tennis ball or an elephant?

Example Question #332 : High School Physics

Two skaters push off of each other in the middle of an ice rink. If one skater has a mass of  and an acceleration of , what is the mass of the other skater if her acceleration is ?

Possible Answers:

Correct answer:

Explanation:

For this problem, we'll use Newton's third law, which states that for every force there will be another force equal in magnitude, but opposite in direction.

 

This means that the force of the first skater on the second will be equal in magnitude, but opposite in direction:

 

 

Use Newton's second law to expand this equation.

 

 

We are given the acceleration of each skater and the mass of the first. Using these values, we can solve for the mass of the second.

 

 

Notice that the acceleration of the second skater is negative. Since she is moving in the opposite direction of the first skater, one acceleration will be positive while the other will be negative as acceleration is a vector. From here, we need to isolate the mass of the second skater.

 

 

 


 

Example Question #333 : High School Physics

Basic Computational 

 

A rock strikes a window with  of force. How much force does that window exert on the rock?

 

Possible Answers:

 We need to know how long the two were in contact to solve

 We need to know the mass of the rock to solve

Correct answer:

Explanation:

Newton's third law states that when one body exerts a force on another body, the second body exerts a force equal in magnitude, but opposite in direction, on the first body.

 

Mathematically, this process can be written as:

 

 

Since the rock exerts  of force on the window, then the window must exert  of force on the rock.

 

Example Question #334 : High School Physics

A boy falls out of a tree and hits the ground with  of force. How much force does the ground exert on the boy?

Possible Answers:

We must know the mass of the boy to solve

Correct answer:

Explanation:

Newton's third law states that when one object exerts a force on another object, that second object exerts a force of equal magnitude, but opposite in direction on the first.

 

That means that:

 

 

Using the value from the question, we can find the force of the ground on the boy.

 

 

 

 

 

Example Question #31 : Newton's Laws

Conceptual

 

If you exert a force F on an object, the force which the object exerts on you will

 

Possible Answers:

Always be F

 Depend on the relative masses of you and the object

Depend on whether or not the object is moving

 Depend on whether or not you are moving

Correct answer:

Always be F

Explanation:

According to Newton’s 3rd Law of Motion, the force that is exert by object A onto object B is equal in magnitude and opposite in direction to the force that is exerted by object B onto object A.  It is not dependent on the mass or motion of the object.

 

Example Question #336 : High School Physics

An object of mass m sits on a flat table.  The Earth pulls on the object with force mg, which is the action force.  What is the reaction force?

Possible Answers:

 The object pulling upward on the earth with force 

 The object pushing down on the table with force 

 The table pushing down on the floor with 

The table pushing up on the object with force 

Correct answer:

 The object pulling upward on the earth with force 

Explanation:

There is a common misconception that the force that Earth pulls on the object is balanced by a reaction force of the table pushing up on the object.  Though it is true that the magnitude between these forces are the same, they are not considered action reaction pairs.  

 

According to Newton’s 3rd law the force with which object A acts on object B is equal and opposite to the force that object B acts on object A.  In this case the initial force is the Earth pulling on the object.  Therefore the pair would be the object pulling on the Earth.

 

If we consider the table we are adding a third object to the mix, which cannot be an action reaction pair.  Therefore the normal force and the force of gravity are never considered action reaction pairs as they are two different forces acting on the same object.

 

Example Question #337 : High School Physics

A ball is suspended from the ceiling by means of string.  The Earth pulls downward on the ball with its weight force of .  If this is the action force, what is the reaction force?

Possible Answers:

The ceiling pulling upward on the string with a  force

 The string pulling downward on the ceiling with a  force

 The ball pulling upward on the earth with a  force

The string pulling upward on the ball with a  force

Correct answer:

 The ball pulling upward on the earth with a  force

Explanation:

There is a common misconception that the force that Earth pulls on the object is balanced by a reaction force of the string pulling upward on the object.  Though it is true that the magnitude between these forces are the same, they are not considered action reaction pairs.  

 

According to Newton’s 3rd law the force with which object A acts on object B is equal and opposite to the force that object B acts on object A.  In this case the initial force is the Earth pulling on the object.  Therefore the pair would be the object pulling on the Earth.

 

If we consider the table we are adding a third object to the mix, which cannot be an action reaction pair.  Therefore the tension force and the force of gravity are never considered action reaction pairs as they are two different forces acting on the same object. 

Example Question #338 : High School Physics

 Two skaters push off of each other in the middle of an ice rink. If one skater has a mass of  and an acceleration of , what is the acceleration of the other skater if her mass is ?

Possible Answers:

Correct answer:

Explanation:

For this problem, we'll use Newton's third law, which states that for every force there will be another force equal in magnitude, but opposite in direction.

 

This means that the force of the first skater on the second will be equal in magnitude, but opposite in direction:

 

 

Use Newton's second law to expand this equation.

 

 

We are given the mass of each skater and the acceleration of the first. Using these values, we can solve for the acceleration of the second.

 

 

From here, we need to isolate the acceleration of the second skater.

 

 

 

 

 

 

Notice that the acceleration of the second skater is negative. Since she is moving in the opposite direction of the first skater, one acceleration will be positive while the other will be negative as acceleration is a vector.

 

 

Example Question #1 : Newton's Law Of Universal Gravitation

An asteroid with a mass of  approaches the Earth. If they are  apart, what is the asteroid's resultant acceleration?

Possible Answers:

Correct answer:

Explanation:

The relationship between force and acceleration is Newton's second law:

We know the mass, but we will need to find the force. For this calculation, use the law of universal gravitation:

We are given the value of each mass, the distance (radius), and the gravitational constant. Using these values, we can solve for the force of gravity.

 

Now that we know the force, we can use this value with the mass of the asteroid to find its acceleration.

 

 

Example Question #2 : Newton's Law Of Universal Gravitation

Two asteroids exert a gravitational force on one another. By what factor would this force change if one asteroid doubles in mass, the other asteroid triples in mass, and the distance between them is quadrupled?

Possible Answers:

Correct answer:

Explanation:

The equation for the force of gravity between two objects is:

 

Using this equation, we can select arbitrary values for our original masses and distance. This will make it easier to solve when these values change.

 

 is the gravitational constant. Now that we have a term for the initial force of gravity, we can use the changes from the question to find how the force changes.

We can use our first calculation to see the how the force has changed.

 


 

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