# Homework Help: A Rocket was launched directly upwards from rest.

1. Dec 29, 2011

### heartOFphysic

Hi, I have this particular question, to which i think my method is correct, though the answer from the book which this question is in gives different to mine. Please! any assistance will be very much appreciated!

Question:

A Rocket was launched directly upwards from rest. Its motors operated for 30s after it left the launch pad, providing it with a constant vertical acceleration of 6.0m/s during this time. Its motors then switched off.

Calculate its velocity.

My method:
Take upwards as + and g = 9.8

then u= 0 t= 30 a= -9.8+6 = -3.8

since V= u +at we get -3.8 X 30 = -114 = 114m/s!

BUT THE BOOK GIVES THE ANSWER AS 180m/s. (This means they've taken acceleration as 6! But surely acceleration due to gravity is acting downwards, so we need to calculate a resultant acceleration first!)

thanks

2. Dec 29, 2011

### Staff: Mentor

Re: Rocket

Presumably, the 6 m/s2 is the resultant acceleration, including the effect of gravity. (It wouldn't be much of a rocket if the net acceleration were downward!)

3. Dec 29, 2011

### heartOFphysic

Re: Rocket

Ahhhh I get it now! so the motor is providing an acceleration of 15.8m/s^2!!!

hence resultant acceleration is 15.8-9.8= 6!

I wouldn't have presumed that was the case, to me 6m/s^2 was the genuine acceleration the motor provided.

Thank you for your help!