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# Apply Newton's first law to each of the following situations.

In which situations can you conclude that the object is undergoing a net interaction with one or more other objects? (Select all that apply.
1)A car travels at constant speed around a circular race track.
2)A book slides across the table and comes to a stop.
3)A hydrogen atom remains at rest in outer space.
4)A proton in a particle accelerator moves faster and faster.
5)A spacecraft travels at a constant speed toward a distant star.

Thank you in advance.

### 2 Answers by Expert Tutors

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Caleb M. | Mathematics TutorMathematics Tutor
5.0 5.0 (102 lesson ratings) (102)
3
Recall that an object experiencing a net force undergoes an acceleration. So we only need look for bodies slowing down or speeding up:

1) The car may be driving at constant speed but it continues to drive in a circle. Something has to be 'drawing' it to the center of the track for this to continue (just as the planets orbit the sum) so there must be some 'external force'.

2) The book is moving and doesn't come to a stop via magic. So something acted on the book to slow it down to a stop. So there is net acceleration here (the kinetic friction from the table).

3) This one is very simple, nothing is moving!

4) The proton doesn't accelerate by dark magic from beyond! Something has to be acting on it to accelerate it. So there is some net acceleration.

5) The craft is neither speeding up nor slowing down. So there is no net acceleration here.

*Note: Just because there are no net accelerations does NOT mean there are not forces at work. For example, imagine you and I locking arms in an arm-wresting contest. We are both trying to give each others arm an angular acceleration to win. But if we are at a stalemate, this is because the net forces cancel - we are equally strong. But despite this, we are applying forces. It's just that here they are balanced.

### Comments

In situation (1), the car is under acceleration because its direction is changing, even while moving at constant speed.  It is under the influence of a centripetal force that keeps it on the circular path.  The angular speed is constant, and there is no angular acceleration.  But there is linear acceleration, since the motion is not one of constant speed in a straight line.  Its velocity is changing, since its direction is changing, even if its speed is not changing.
Your answer about 3 and 5 only apply to Newton's First Law is not correct.
My answer is correct.  Situation (3) involves a body at rest remaining at rest.  It is not accelerating, and therefore its state of motion is described by Newton's first law.  Similarly, situation (5) involves a body moving at a constant speed in a straight line.  We assume that since the star is distant, the body is not under the influence of the star's gravitation, and hence not under acceleration.  This is a first law situation.  The other situations involve acceleration, and hence the presence of a net external force.  Those are second law situations.  You have correct answers with adequate explanations from both Caleb and me.

I noticed down-votes for the answers from Caleb and me.  They are not justified.
With all due respect. Why does Webassign count this as wrong. I chose the answers that you said and it showed it as wrong.
I do not know what criteria Webassign uses for grading, so I cannot explain why it shows this as wrong.  But let us look at Newton's first 2 laws:

(1) If no net external force is applied to a body, it will remain at rest if already at rest, or it will remain in a state of motion of constant speed in a straight line if already moving.

(2) If a body of mass m has an acceleration a, the net external force F applied to the body is F = ma.  (And remember acceleration can be a change in speed, or a change in direction, or both.)

Now compare these statements of Newton's first and second laws to the 5 situations in the problem, and see that the first law describes the situations in (3) and (5), while the second law describes the situations in (1), (2), and (4).
I am deeply sorry for giving you and Caleb thumbs down. I gave both of you an extra thumbs up. I truly apologize.
Arturo O. | Experienced Physics Teacher for Physics TutoringExperienced Physics Teacher for Physics ...
5.0 5.0 (66 lesson ratings) (66)
1
Newton's first law applies to (3) and (5), where a body at rest remains at rest, or a body in motion continues to move in a straight line at constant speed.  The situations of (1), (2), and (4) are described by Newton's second law, since either the speed or direction of motion (or both) are changing.  Those are examples of accelerated motion under the influence of a net external force.  Hence, there is another object or a field or some other agent present that is applying the force that causes the acceleration.

### Comments

Only numbers 3 and 5 are not correct. I checked by submitting one time but it is incorrect.
I am deeply sorry for giving you and Caleb thumbs down. I gave both of you an extra thumbs up. I truly apologize.