Confusing Mechanics Question: Velocity of Rocket on Touchdown

In summary, the textbook authors were using different conventions for positive and negative directions. The "positive up" convention is positive when looking towards the ground and negative when looking upwards, while the "positive down" convention is the opposite.
  • #1
Falley
4
0
So,while learning mechanics,I found that most questions use g=-9.8 ,but when calculating the velocity the instant a rocket touches the ground,the solution instead used g=9.8,which is very confusing.Other questions of similar workings used g=-9.8,anybody care to help?
 
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  • #2
It depends on which direction you define as "up". As long as you are consistent, it doesn't matter which you use.
 
  • #3
But for the same question,part a),b)c) a n b used -9.8 while c used 9.8
 
  • #4
In every problem we get to choose whether we consider forces, velocities, and accelerations to be positive when they're in the upwards direction and negative when they're in the downward direction, or the other way around. Gravity pulls downward, so it will be negative if you use the "positive up" convention, positive if you use the "positive down" convention.

Either way works just fine - you just have to be consistent throughout the problem.
 
  • #5
Falley said:
But for the same question,part a),b)c) a n b used -9.8 while c used 9.8

Most likely your textbook author was most comfortable thinking about parts a and b one way and part c the other way. Once you get used to it, you'll find yourself picking whichever one feels most natural for a given problem or subproblem without even thinking about it.

Or maybe he did it in purpose, just to keep you on your toes... Recognizing which sign convention is being used and being able to handle either one is an important skill and exercising it might have been the point of this problem.

Or maybe he was just careless. It happens :smile:
 
  • #6
Post the exact question otherwise we are guessing.
 
  • #7
Example 3,the rocket
 

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  • #8
Both are meaningless without units... :)
 
  • #9
Falley said:
Example 3,the rocket
That is just a typo. The equation, as written, doesn't have the solution 25.9 s. The solutions for +9.8 m/s^2 are both negative. Only -9.8 m/s^2 gives a positive solution and that soluiton is 25.9 s.
 
  • #10
In case it's not obvious... Their approach is far more complicated than necessary. They are working out the impact velocity starting from the point where the rocket motor shuts off on the way up. It's possible to do that but it's much easier to assume it just falls from the max height which you worked out in part (b).
 
  • #11
Thanks a lot,cleared up a lot of things for me
 

Related to Confusing Mechanics Question: Velocity of Rocket on Touchdown

What is the velocity of a rocket on touchdown?

The velocity of a rocket on touchdown depends on various factors such as the speed at which it was launched, the force of gravity, and the atmospheric conditions. It is also affected by the type of landing, whether it is a soft or hard landing.

How is the velocity of a rocket on touchdown calculated?

The velocity of a rocket on touchdown can be calculated using the equation v = u + at, where v is the final velocity, u is the initial velocity, a is the acceleration due to gravity, and t is the time it takes for the rocket to touchdown.

Why is the velocity of a rocket on touchdown important?

The velocity of a rocket on touchdown is important because it determines the impact force on the rocket and the landing surface. A high velocity can cause damage to the rocket and the surrounding area, while a lower velocity can result in a smoother landing.

What factors can affect the velocity of a rocket on touchdown?

The velocity of a rocket on touchdown can be affected by the initial launch speed, the angle of descent, the force of gravity, and any external forces such as wind or air resistance. The weight and design of the rocket can also play a role in its velocity on touchdown.

How can the velocity of a rocket on touchdown be controlled?

The velocity of a rocket on touchdown can be controlled by adjusting the initial launch speed and angle, as well as using parachutes or other landing mechanisms to slow down the descent. Additionally, accurate calculations and careful planning can help ensure a safe and controlled landing velocity.

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