# Homework Help: Physics B question.

1. Nov 22, 2009

### cyspope

1. The problem statement, all variables and given/known data
An astronaut lands on an unknown planet with a radius of 3560 m. When she jumps upward with an initial speed of 3.00m/s she rises to a height of 0.50 m. What is the mass of the planet?

2. Relevant equations
d = rt
x = x0 + v0t + $$\frac{1}{2}$$at2
M = gr2 / G

3. The attempt at a solution
0.5 m / (3.0m/s) = $$\frac{1}{6}$$s
0.5 = 0 + 3$$\frac{1}{6}$$s + $$\frac{1}{2}$$a$$\frac{1}{6}$$2
0.5 = $$\frac{1}{2}$$ + $$\frac{1}{2}$$a$$\frac{1}{36}$$
a = 0 m/s2??????

2. Nov 22, 2009

### Delphi51

You need to get the value of the acceleration from the information given about the motion of the astronaut. Look for an accelerated motion formula with distance but not time in it. Put in the numbers and solve for a.

Your calc getting 1/6 second looks like a d = vt, which does not apply to accelerated motion.

3. Nov 23, 2009

### cyspope

In order to find the acceleration(gravity of the planet in this case) of the astronaut, I used following formula.

vf2 = v02 + 2a$$\Delta$$X

If I solve for 'a', then I get

a = (vf2-vo2 / 2$$\Delta$$X

I pluged in the numbers,

a = (9m/s) / 2(0.5 m) = 9 m/s2

which doesn't make sense because the radius of the planet is only 3560 m.