# Homework Help: Forces acting on a sky-diver during skydiving

1. Dec 1, 2013

### AlfieD

This is really simple but seems to be confusing me (mainly the third diagram in the picture). The question is to label ALL the forces (arrows) in ALL the diagrams with either 700N, more than 700N, or less than 700N. It should be noted that the sky-diver weighs 700N. My answers that you can see were me misreading the question and I think I was calculating his weight. Given a proper read of the question, mg new answers would be (from top arrow to bottom arrow): more than 700N, 700N, 700N, not sure (third diagram 1st arrow), not sure (2nd), 700N, 700N.

Thanks in advance, sorry for the incompetency.

2. Dec 1, 2013

### Simon Bridge

Given a proper read of the question, mg new answers would be (from top arrow to bottom arrow): more than 700N, 700N, 700N, not sure (third diagram 1st arrow), not sure (2nd), 700N, 700N.

OK - lets make sure I understand you.

Diagram 1 shows one arrow pointing down as the skydiver starts out on the jump.
You say this is "more than 700N"

Diagram 2 shows two arrows as the skydiver reaches constant speed.

Diagram 3 shows two arrows as the skydiver's shute first opens.

Diagram 4 shows two arrows as the skydiver reaches constant speed.

... where does the downward force come from in each case?
... is the skydiver accelerating, decelerating, or travelling at a constant speed, as he falls?

Complete the following sentenses by placing the words "bigger than", "smaller than", or, "the same size as" in the space provided:

If accelerating, then the up arrow is _______ than the down arrow.
If decelerating, then the up arrow is _______ than the down arrow.
If constant speed, then the then the up arrow is _______ than the down arrow.

3. Dec 2, 2013

### AlfieD

1) Smaller than
2) Bigger than
3) The same size as

?

So would my new answers be right. If so, were the forces for the third diagram (which I was unsure of), bigger than 700N (up arrow), and then smaller than 700 N (down arrow)?

4. Dec 2, 2013

### Simon Bridge

1,2,3 are correct.

It has a special name - what is it called?

5. Dec 10, 2013

### AlfieD

The diver's weight.

Thanks, I get it now.

6. Dec 10, 2013

### Simon Bridge

Well done :)

For people who google here later and are still confused:
The downwards arrow in each diagrams does not change length.
Only the upwards arrow changes.

downwards force W: W=700N in all the pictures.
upwards force F: F=W for constant falling speed, F>W when decelerating, and F<W when accelerating.

the free body diagram should give you W-F=ma for positive downwards.
the question is testing your understanding of this relationship.