Finding Velocity with Kinetic Energy.

[kencamarador]

Homework Statement

I have to use energy conservation to solve this. So i cannot solve with kinematics

"On a windless day, the engine of a 250 g toy rocket exerts 30N of thrust straight upward until it burns out at a height of 165 m"

How fast is the rocket moving at 165m?

Homework Equations

Ek=mv^2 / 2
Em=eg+ek
Eg=mgh

The Attempt at a Solution

First i found my acceleration.

A= f/m
A= (30- 9.8 x .25) / 0.25
=110.2

Then subbed into

Vf^2 = 0 + 2(110.2)x165
190.1

But my teacher told us to use the law of conservation...

 

[rock.freak667]

Well the work done in moving the rocket 165 m will be equal to the kinetic energy of the rocket at 165 m.

How would you find the work done by the force of 30 N in moving 165 m?

 

[kencamarador]

Well the work done in moving the rocket 165 m will be equal to the kinetic energy of the rocket at 165 m.

How would you find the work done by the force of 30 N in moving 165 m?

Mutliply 30 by 165 which is 4950

oh... Then solve for v

 

[vela]

You need to take into account gravity.

 

[Nickie]

I would like to clarify that the approach of using energy conservation to solve this problem is valid and can provide accurate results. The equations used are correct and the calculation of acceleration is also accurate. However, using kinematics equations would also be a valid approach to solve this problem.

Kinematics and energy conservation are two different approaches to solving problems in physics. While kinematics deals with the motion of objects and their position, energy conservation deals with the transfer and transformation of energy within a system. Both approaches can provide valuable insights and solutions to different problems.

In this specific problem, using energy conservation allows us to consider the energy changes within the rocket as it moves upwards. This approach takes into account the work done by the engine (30N of thrust) and the potential energy gained by the rocket due to its position at a certain height. Using the equations provided, we can accurately calculate the final velocity of the rocket at 165m.

On the other hand, using kinematics equations would involve considering the initial velocity of the rocket, the acceleration due to gravity, and the displacement. This approach would also provide an accurate solution to the problem.

In conclusion, as a scientist, it is important to understand and be familiar with different approaches to solving problems in physics. Both energy conservation and kinematics are valid methods and can be used to solve this problem.