# Homework Help: Skydiver physics homework

1. May 18, 2005

### Logistics

This is my 3rd practice question that I don't know how to do :( Hoping peole can help me out, if someone could even work it out, would be greatly appriciated.

A skydiver, weighing 70kg, jumps from an aeroplane at an
altitude of 700 metres and falls for T1 seconds before pulling
the rip cord of his parachute. A landing is said to gentle if
the velocity on impact is no more than the impact velocity of
an object dropped from a height of 6 metres. The distance

that the skydiver falls during t seconds can be found from New-
ton’s Second Law, F = ma. During the free fall portion of
the the jump, we will assume that there is essentially no air
resistance, so F = −mg where g = 9.8ms^−2 and m = 70kg.
After the parachute opens, a significant drag term due to the
air resistance of the parachute affects the force F, causing the
force to become F = −mg − kv where v is the velocity and
k = 110kg/sec is a drag coefficient.

(a) Find the range of times T1 at which the rip cord can be
pulled for a gentle landing.

(b) Find the height after T1 seconds of free-fall.

Last edited by a moderator: May 11, 2014
2. May 18, 2005

### HallsofIvy

You've given a pretty detailed explanation of HOW to do the problem- all that's missing is writing down the approriate formulas and then doing the arithmetic.

How is distance fallen related to acceleration in free fall?

What is the total force on the skydiver after the the parachute opens?

More than that we can't say until you show us what you have tried and what knowledge you have. If you can do differential equations, this is pretty straightforward. If not, then you would have to have been given appropriate formulas relating force, acceleration, speed, and distance. What formulas do you have?

3. May 19, 2005

### Logistics

Well we got this in maths. And we have studied DE's. I also study physics. So pick your method / formulas.

If I solved what v is how would that help me ?

4. May 19, 2005

### HallsofIvy

If you solve for v and d for the diver in free fall (as a function of t of course), then you know how fast he is going and how high he is when his parachute opens (t1). Using those as initial values, you can solve for his speed and height at any time t after the parachute opens. Then find his speed when he hits the ground (what do you consider a "gentle landing"?) and any time t.

5. May 19, 2005

### learningphysics

Is an analytical solution possible? When I find an equation for T1 it includes exponential as well as polynomial terms... It seems like you need to graph it to solve for T1.

6. May 20, 2005

### Logistics

learningphysics, I think you're right about the graph to find the possible values.

I'll see what I can do, damn taugh question for me

7. May 23, 2005

### Logistics

Solved the question, thanks for the replies