# Homework Help: Speed due to gravity

1. Jun 1, 2010

### ghostbuster25

This is a revision question for a test ive got. Just want to check my answer.

An asteroid with mass of 1.8*1015kg and radius of 6.7km attracts a spacecraft that has come into its gravitational influence. What speed will the spcaecraft crash into the surface. intial speed of spacecraft is negilable.

its a multiply choice and i choose it is impossible to calculate without knowing the mass of the spacecraft.

the equation i would have used is -Gm1m2/r

this right? or is there a way of working out the speed without knowing the mass?

thanks

2. Jun 1, 2010

### ehild

Yes, it is a way to find the speed without knowing the mass of the spacecraft.

You said you would have used the equation -Gm1m2/r, but its is not an equation without the other side. What is it equal to?

ehild

3. Jun 1, 2010

### ghostbuster25

well i worked out the gravitaional pull of the asteroid by using Gm1/r^2 and found it to be 0.0026757m/s which makes sense as its a small object. But im not sure where to go from there as i dont know a, the distance the spacecraft is and b, the mass of the spacecraft so cant use the normal a=f/m

4. Jun 1, 2010

### Cyosis

Hint: Energy conservation: E(infinity)=E(distance at which it impacts the asteroid)

5. Jun 1, 2010

### ghostbuster25

lol im really lost now

6. Jun 1, 2010

### Fightfish

The total energy (gravitational and kinetic) of the system remains conserved throughout the interaction ie. total initial energy = total final energy.
Basically, use the energy approach in tackling this problem.