Notice in the question. It is saying when air resistance is not ignored.?

Question

A rock is suspended from a string; and it moves downward at constant speed. Which one of the following statements is true concerning the tension in the string if air resistance is not ignored?


1)The tension is greater than the weight of the rock.


2) The tension points downward.


3) The tension is less than the weight of the rock.


4) The tension is zero newtons.


5) The tension is equal to the weight of the rock.

Answer 1

The usage of "suspended" brings a technical error, because suspended objects dont do free falls. A better word would have been "tied".

# 3 Would be the best option in that case. Not ignoring air resistance suggests terminal velocity and since the terminal velocity of the string is much lower than the rock, the rock would be exerting some force of the string. This will create a tension and this is less then the weight of the rock.

Answer 2

You have three forces acting on the rock:

1) Its weight W (downward);
2) Tension T in the string (upward);
3) Air resistance R (upward, because the rock is moving down).

Since the rock is not accelerating, it must be that these three exactly balance each other. That is:

downward = upward
W = T + R

So consider each choice:

"1)The tension is greater than the weight of the rock."
Nope, because T = W–R. So T is less than W.

"2) The tension points downward."
Nope, because tension is always in the direction from the object toward the string.

"3) The tension is less than the weight of the rock."
Yes, because T = W–R. So T is less than W.

"4) The tension is zero newtons."
This actually could be true. You could have a rock falling at its terminal velocity, with a zero-tension string just flapping along behind it. But since they used the word "suspended" in the question, I'll take that as implying that there's some tension in the rope. It's not implied by the equations, however.

"5) The tension is equal to the weight of the rock."
Nope, because T = W–R. So T is less than W.

Answer 3

I'd assume it would mean don't ignore the resistance from air that would apply an upward force on the rock.. ruling out #5

Answer 4

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