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

Question

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.

Answer 1

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. 

Answer 2

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.