EK Physics 1001 #202

[QuiSait]

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The question has a drawing of a curvy line and says:
An object moves at a constant speed along the path shown below. Which of the following must be true?

The answer is:
There is a changing net force acting on the object. At all times, the force is in a direction perpendicular to the motion of the object.

(all of the wrong answers are permutations of the opposite of the correct answer)

The explanation is:
Since the speed of the object did not change, no work was done on the object, which means there was no net force in the direction of the displacement. However, any turn represents an acceleration, which requires a net force. Thus, all the net force must have been perpendicular to the motion at all times.

I actually got this one right, but I don't understand their explanation. And I sort of guessed at the perpendicular part.

My questions:
- why does the speed of the object not changing mean that no work was done? Because of Newton's First Law? And why does that mean there was no net force in the direction of the displacement?
- and why must the net force have been perpendicular to the motion at all times?

Thank you!

 

[PiBond]

The question has a drawing of a curvy line and says:
An object moves at a constant speed along the path shown below. Which of the following must be true?

The answer is:
There is a changing net force acting on the object. At all times, the force is in a direction perpendicular to the motion of the object.

(all of the wrong answers are permutations of the opposite of the correct answer)

The explanation is:
Since the speed of the object did not change, no work was done on the object, which means there was no net force in the direction of the displacement. However, any turn represents an acceleration, which requires a net force. Thus, all the net force must have been perpendicular to the motion at all times.

I actually got this one right, but I don't understand their explanation. And I sort of guessed at the perpendicular part.

My questions:
- why does the speed of the object not changing mean that no work was done? Because of Newton's First Law? And why does that mean there was no net force in the direction of the displacement?
- and why must the net force have been perpendicular to the motion at all times?

Thank you!

For your first question, there was no change in KE (kinetic energy) and thus, according to the work energy theorem (Work = changeKE), there must have been no work done on the object. There was no net force in the direction of displacement because speed was constant--if there had been a force then speed would have changed
The force must have been perpendicular to the motion at all times because the direction/path of the objection is changing (and there must be a force to cause this) but the speed is not changing so the force isn't in the direction of displacement

 

[QuiSait]

Thanks for the help, that was great. I had some gaps about KE, now it makes sense. Thanks!