High School Physics : Newton's Laws

Study concepts, example questions & explanations for High School Physics

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Example Questions

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Example Question #1 : Newton's Laws

Which of these is NOT an example of inertia? 

Possible Answers:

A satellite rotating in outer space will continue to spin at roughly the same rate

The force a hammer exerts on a nail is the same as the force the nail exerts on the hammer

A construction worker pushes a metal box and notices it is harder make the box start moving than it is to push it at a constant velocity

A passenger is thrown forward when his car collides with a wall

A child riding a merry-go-round lets go and is thrown off the ride

Correct answer:

The force a hammer exerts on a nail is the same as the force the nail exerts on the hammer

Explanation:

Newton's first law says that an object in motion will remain in motion in the same direction unless acted upon by an outside force; an object at rest will remain at rest unless acted upon by an outside force.

In the example of the car hitting the wall, the passenger continues travelling at the same speed that the car was moving directly before impact. He does not stop his forward motion until an outside force (his seatbelt) stops him.

In the example of the satellite, the rotation of the satellite can only change rate if an outside force interferes. Because there are no forces to affect the satellite's spinning, it will continue to do so as per Newton's first law.

In the example of the construction worker, the box was at rest and therefore resisted any change to being at rest. Once it is in motion, it will continue in motion in the same direction. This principle is the same reason why static coefficients of friction are generally greater than kinetic coefficients of friction.

In the example of the child on the merry-go-round, the important part of Newton's first law to recall is that objects will remain in motion in the same direction. The rotational motion of the child will result in a constantly changing velocity in the direction tangent to the edge of the ride. Once the child lets go, she will move in a straight line directly off of the ride.

The hammer and nail example illustrates Newton's third law, but has no bearing on the principle of inertia.

Example Question #1 : Understanding Newton's First Law

While pushing a very heavy box, Derek notices that it was much harder for him to get the box to start moving than it was for him to get the box to continue to move once it had started. This is an example of which mechanics principle?

Possible Answers:

Newton's second law

Law of universal gravitation

Law of frictional force

Newton's first law

Newton's third law

Correct answer:

Newton's first law

Explanation:

This is an example of Newton's first law: an object at rest will remain at rest, and an object in motion will remain in motion in that direction, unless acted upon by an outside force.

Inertia is effectively nature's way of trying to avoid change. This explains why the box is hard to move while it is still; it requires change to get it to move from rest to moving. It is easier to continue motion when it is moving because it requires much less change to keep it moving in the same direction.

Mathematically, this principle dictates that the coefficient of static friction will always be greater than the coefficient of kinetic friction.

Example Question #1 : Understanding Newton's Second Law

Which of the following is not a part of Newton's second law?

Possible Answers:

Displacement

Force

Acceleration

Mass

Correct answer:

Displacement

Explanation:

Newton's 2nd law states . Therefore, all we need is a force, a mass, and an acceleration!

Example Question #2 : Understanding Newton's Second Law

A block is pushed with  newtons of force. What other information do we need in order to find the acceleration of the block?

Possible Answers:

Final velocity

Mass

Work done on the block

Acceleration due to gravity

Initial velocity

Correct answer:

Mass

Explanation:

Newton's second law states that .

If we know the force, , then we only need to know the mass, , in order to find acceleration.

Example Question #3 : Understanding Newton's Second Law

 orange falls from a tree. What is the force of gravity on the orange?

Possible Answers:

Correct answer:

Explanation:

Newton's second law states that:

We are given the mass of the orange and the acceleration; since we are looking at the force due to gravity, the acceleration will be the acceleration due to gravity. Use these given values to calculate the force.

Keep in mind that the force will be negative, since gravity acts in the downward direction.

Example Question #4 : Understanding Newton's Second Law

ball rests on a flat table. What is the normal force exerted on the ball by the table?

Possible Answers:

Correct answer:

Explanation:

Newton's second law allows us to solve for the force of gravity on the ball:

Newton's third law tells us that the force of the ball on the table, due to gravity, will be equal and opposite to the normal force of the table on the ball.

Substitute the equation for force of gravity.

Now we can use the mass of the ball and the acceleration of gravity to solve for the normal force. First, convert the mass to kilograms. Then, use the equation to find the normal force.

Example Question #5 : Understanding Newton's Second Law

Which of these is necessary for there to be a non-zero net force?

Possible Answers:

Non-zero net acceleration

Non-zero net displacement

An equal and opposite reaction

Non-zero net torque

Non-zero frictional force

Correct answer:

Non-zero net acceleration

Explanation:

Newton's second law states that force is a mass times an acceleration.

In order for a force to exist, there must be an acceleration applied to a mass. A force cannot exist on a massless object, nor can it exist without a net acceleration.

Newton's third law states that for every force on an object, there is an equal and opposite force from the object. These force frequently cancel out, however, and produce a net force of zero.

Example Question #1 : Understanding Newton's Second Law

Which of these is not an example of Newtonian mechanics?

Possible Answers:

Correct answer:

Explanation:

Newtonian mechanics apply to all objects of substantial mass travelling at significantly slower than the speed of light.

Newton's law of universal gravitation, Newton's second law, momentum, and the equation for mechanical energy all fall under Newtonian mechanics.

The mass-energy equivalence suggests that mass can change as the speed of an object (such as an electron) approaches the speed of light. Newtonian mechanics assume that mass is constant, and do not apply to objects approaching the speed of light.

Example Question #1 : Understanding Newton's Third Law

If a tennis ball hits a racket with  of force, what is the force of the racket on the ball?

Possible Answers:

Correct answer:

Explanation:

Newton's third law states that every force has an equal and opposite force.

That means that . Since the force of the ball equals , the force of the racket must be equal and opposite.

.

Remember, this force will be negative as it is equal and OPPOSITE. That means it is moving in the opposite direction.

Example Question #2 : Understanding Newton's Third Law

A bat hits a baseball with  of force. How much force does the baseball exert on the bat?

Possible Answers:

Correct answer:

Explanation:

Newton's third law states that when object A exerts a force on object B, object B exerts a force equal in magnitude but opposite in direction on object A.

That means that if the force of the bat on the ball is , then the ball on the bat must be .

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