High School Physics : Understanding Newton's First Law

Study concepts, example questions & explanations for High School Physics

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Example Questions

Example Question #1 : Newton's Laws

Which of these is NOT an example of inertia? 

Possible Answers:

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

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

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

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

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 : Newton's Laws

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 first law

Newton's second law

Law of universal gravitation

Law of frictional force

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 First Law

You are standing in a moving bus, facing forward, and you suddenly fall forward as the bus comes to an immediate stop.  What force caused you to fall forward?

Possible Answers:

Gravity

There is not a force learning to your fall

Force due to friction between you and the floor of the bus

Normal force due to your contact with the floor of the bus

Correct answer:

There is not a force learning to your fall

Explanation:

While the bus in moving forward, you move forward with the bus at the same velocity.

 

When the bus stops, your body continue to move forward with the original velocity at which you had been traveling. Since you are not attached to the bus, you will not stop at the same rate as the bus and will fall forward as your body continues to move.

 

This is the reason why seatbelts are so important as it keeps you fixed to the car so that you undergo the same forces that are acting on the car.

 

Example Question #1 : Newton's Laws

A truck is traveling horizontally to the left.  On the bed of the truck is a crate.  When the truck starts to slow down, the crate on the (frictionless) truck bed starts to slide.  In what direction could the net force be on the crate?

Possible Answers:

No direction.  The net force is zero.

 Horizontal and to the right.

 Straight down (because of gravity)

Horizontal and to the left.

 Straight up (the normal force)

Correct answer:

No direction.  The net force is zero.

Explanation:

While the truck in moving forward, the carte moves forward with the truck at the same velocity.

 

When the trucks stops, the crate continues to move forward with the original velocity at which it had been traveling since there is no friction to stop the crate from moving. Since the crate is not attached to the truck, it will not stop at the same rate as the truck and will slide toward the cab of the truck as the truck slows down.

 

This is the reason why tie-downs are so important as it keeps objects fixed to the truck so that they undergo the same forces that are acting on the truck.

 

Example Question #3 : Understanding Newton's First Law

Conceptual

 

Which of these is NOT an example of inertia?

 

Possible Answers:

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

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

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

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 #4 : 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 third law

Law of frictional force

Law of universal gravitation

Newton's second law

Newton's first 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 #5 : Understanding Newton's First Law

In the absence of an external force, a moving object will

Possible Answers:

Stop immediately

 Move with constant velocity

 Go faster and faster

 Slow down and eventually come to a stop

Correct answer:

 Move with constant velocity

Explanation:

According to Newton’s First law, an object in motion will remain in motion until there is an external force acting on this.  This is observable in the real world as marbles will continue to roll until they are slowed down by friction or stopped by a wall or a person or some other external force.

 

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