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:

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

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

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 #2 : 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

Law of frictional force

Newton's third law

Newton's second law

Law of universal gravitation

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.

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