# Homework Help: Initial Velocity of a Rocket

1. Jun 9, 2009

### python1

1. The problem statement, all variables and given/known data
I need to find the initial velocity of a rocket using the method a=$$\Delta$$v/$$\Delta$$t. Given a situation where $$\Delta$$t = 4 seconds. The force of gravity in this case is -10(m/s)

2. Relevant equations
After I get $$\Delta$$v how do I turn that into the initial velocity?

3. The attempt at a solution
a=$$\Delta$$ v/$$\Delta$$ t
-10=$$\Delta$$ v / 4 seconds
$$\Delta$$ v = -40 (m/s)

2. Jun 9, 2009

### LowlyPion

Welcome to PF.

What you found was the speed of an object dropped after 4 seconds ... or if you are talking about a rocket, the speed in accelerating from rest at 10 m/s2.

The acceleration of gravity while of interest doesn't say anything about the force the propellant delivers to the motion of a rocket.

3. Jun 9, 2009

### python1

This is just after launch so I see no need to worry about any force acting on the rocket while it's in the air other than gravity pulling it down.

4. Jun 9, 2009

### drizzle

can you type the full question please, it sounds fuzzy like this

5. Jun 9, 2009

### python1

A rocket has been in the air for 4 seconds from the instant it was launched to when it hit the ground. The force of gravity can be rounded to 10(m/s). Using the method a= delta v / delta t, find the initial velocity for the rocket.

6. Jun 9, 2009

### drizzle

you mean 10 (m/s^2)

wouldn't that motion of the rocket be a bow like motion [projectile motion]since it didn't mention that it was verticly launched!

if so work on the vertical axis, where the final velocity=0 and the intial one v [as you did], then try to find the distance from start point to hit point to get v horizontal which is constant [not accelerated] then add both to get the initial velocity [v_i=squareroot(v_v^2+v_h^2)]

7. Jun 9, 2009

### LowlyPion

OK. So it is not about the rocket at all except that it is a projectile after launch?

In which case the usual rules apply.

V = Vo - a*t = Vo - g*t = Vo - 10*t on the way up and then again on the way down.

With that in mind, then by symmetry it takes 2 seconds up, and 2 more down.

At 2 seconds to max height, then initial velocity is ...