Rocket Acceleration: Calculating Speed and Altitude

[mullets1200]

Homework Statement

A 1000kg weather rocket is launched straight up. The rocket motor provides a constant acceleration for 16s, then the motor stops. The rocket altitude 20s after launch is 5100m. There is no air resistance.

Questions:
What was the rocket's acceleration during the first 16s?

What is the rocket's speed as it passes through a cloud 5100m above the ground

 

[pgardn]

Homework Statement

A 1000kg weather rocket is launched straight up. The rocket motor provides a constant acceleration for 16s, then the motor stops. The rocket altitude 20s after launch is 5100m. There is no air resistance.

Questions:
What was the rocket's acceleration during the first 16s?

What is the rocket's speed as it passes through a cloud 5100m above the ground

Equations?

And your attempts with them?

 

[mullets1200]

Equations?

And your attempts with them?

Equations:
I think you use the equation: r=Vit+.5a(t)²

Im not sure where to start though that is the problem

 

[pgardn]

Equations:
I think you use the equation: r=Vit+.5a(t)²

Im not sure where to start though that is the problem

You are going to need your other kinematic equations.

I would start by looking at the last 4 seconds of the rockets trip (when the only force acting on the rocket is gravity thus a = g = -9.8m/s/s) because the engine has cut off. So when the engine cuts off at 16 seconds you have reached your maximum velocity. After that, during the last 4 seconds the rocket will be slowing down (accelerating down). At the end of this 4 seconds the rocket will be going 0 m/s presumably as it has reached its maximum height.

So why not find the velocity of the rocket when the engine cut off. You got a = g, you got t, and you got Vf = 0 m/s ... find Vo which will be the velocity when the engine cuts off...

Then look at the first 16 s during which the rocket is accelerating up. You have t, you have Vf from the above (it is really Vo from the above), you have Vo = 0 m/s (I will assume the rocket started from rest from the lauch pad) and solve for a.

This is one way to go about it.