Kinematics (distance, speed, velocity, acceleration)

[pttest]

Homework Statement

A rocket blasts off and moves straight upward from the launch pad with constant acceleration. After 3.2 s the rocket is at a height of 85 m. What the magnitude of the rocket's acceleration? What is the direction of the rocket's acceleration (upward / downward)?
What is its speed at this time?

Homework Equations

v=d/t and v=gt (since it moves straight upward)

The Attempt at a Solution

v=85/3.2=26.6m/s
therefore g=v/t => 26.6/3.2= 8.3m/s^2
But the answer is wrong. Could someone help me with it?

Thanks in advance

 

[LowlyPion]

Homework Statement

A rocket blasts off and moves straight upward from the launch pad with constant acceleration. After 3.2 s the rocket is at a height of 85 m. What the magnitude of the rocket's acceleration? What is the direction of the rocket's acceleration (upward / downward)?
What is its speed at this time?

Homework Equations

v=d/t and v=gt (since it moves straight upward)

The Attempt at a Solution

v=85/3.2=26.6m/s
therefore g=v/t => 26.6/3.2= 8.3m/s^2
But the answer is wrong. Could someone help me with it?

Thanks in advance

Since it is a rocket you want to focus on its acceleration which is net of gravity.

When you are given time and distance and want acceleration ...

Isn't it

x = 1/2*a*t²

 

[pttest]

oh ok...

Is the direction of rocket's acceleration downward?

Thanks in advance

 

[LowlyPion]

oh ok...

Is the direction of rocket's acceleration downward?

Thanks in advance

No. The problem statement describes an upward direction of acceleration. Earth's gravity is downward, but the rocket engine is delivering net upward thrust so long as it burns.

At the end of the burn there will be a net downward acceleration even though its velocity continues upward for a while.