# Homework Help: Average force of a rocket

1. Aug 25, 2014

### brake4country

1. The problem statement, all variables and given/known data

A rocket with a mass of 7.2 x 10^4 kg starts from rest in outer space and fires its thrusters until it is moving with a velocity of 100 m/s. What was the average force on the rocket due to the thrusters?

2. Relevant equations

1/2 mv^2 = Fxd

3. The attempt at a solution

I attempted this problem using conservation. However, I keep getting an answer with "d" yet I am unable to solve for d. Can anyone point me in the right direction?

2. Aug 25, 2014

### Orodruin

Staff Emeritus
I suspect the problem creator wants the average force per time unit. Do you know of some physical observable which depends on force and time? (And perhaps one that is also conserved in absence of forces)

Edit: upon rereadin, the time is not specified either. Is the problem formulation exactly as stated?

3. Aug 25, 2014

### brake4country

That would be acceleration?

4. Aug 25, 2014

### Orodruin

Staff Emeritus
Acceleration is only dependent on force and mass. As stated, the problem is only solvable if we assume a constant force, which seems a bit confusing since the problem mentions "average force". Had you been given the time it took to accelerate the rocket, things would have been different.

5. Aug 25, 2014

### vela

Staff Emeritus
You don't have enough information to solve the problem. Is that the complete problem statement as given to you?

6. Aug 25, 2014

### brake4country

No, the problem does not give a time. However, if we use impulse = mv = Ft, then yes, if given the time then Force could be solved. It appears that this problem does not have enough information to solve. How do we know that there is not enough information to solve this problem?

7. Aug 25, 2014

### Staff: Mentor

You'll need a time or a distance over which the acceleration occurs. Does the question specify one of these?

Even the tiniest force will eventually push a craft to any given speed, if that force is applied for long enough.

8. Aug 25, 2014

### brake4country

I see. Also, I understand now that if a problem omits both time and distance there won't be enough info. to solve. This is true for the kinematic equations. Thank you!