# Homework Help: Can someone help me please - Velocity and GPE and Forces

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1. Sep 5, 2016

1. The problem statement, all variables and given/known data

Explain why the velocity at time of impact was much higher than the average velocity

Explain why the actual gravitational potential energy will in reality be much higher than this

Explain why increasing the time of impact would mean the rocket was less likely to get damaged upon impact

Force = Change in momentum/time taken for change

2. Relevant equations

3. The attempt at a solution

Explain why the velocity at time of impact was much higher than the average velocity
The velocity at the time of impact was higher than the average because

Explain why the actual gravitational potential energy will in reality be much higher than this
The GPE will be much higher in reality as there is

Explain why increasing the time of impact would mean the rocket was less likely to get damaged upon impact
Force = Change in momentum/time taken for change
Increasing the time of impact (time taken for change) increases the time taken to change the momentum which reduces the forces. It will take a lot of force to damage the rocket, so reducing the force will damage the rocket less.

2. Sep 5, 2016

### BvU

Hello Dz,

Here at PF we will try to help you as best we can, but you have to make it possible for us to do so. There are some guidelines that can help you post successful threads. (here: re-read your writing before you post and try to imagine what someone who tries to help you needs).

In both your threads the problem statement is incomplete and rewriting in bold face doesn't make it clearer to us helpers.

What on earth (or in space ) is this about? The full story (literally if you can't come up with a concise and complete abstract)

3. Sep 5, 2016

### David Lewis

How does the velocity of a free falling object vary with respect to time? It may help to sketch a rough velocity versus time graph.

4. Sep 5, 2016

You are a Physicist who works for NASA and your job is to evaluate the crash landing of a test rocket. Complete a summary assessment of the rocket’s descent and landing using the data below. You will then need to write a formal report. - Task (I've done most of it)

Mass of the Rocket = 1500kg
Duration of landing impact = 0.04 s
Velocity at time of impact = 175 m/s
Distance travelled = 30km
Time taken to complete journey = 6 minutesAcceleration due to gravity = 9.8 m/s2

Calculations

Average velocity of the rocket’s journey (m/s)
Average velocity = Initial velocity+Final velocity/2
Final velocity = 175 m/s
Initial velocity = u=v-at → u=175-(9.8*360) = -3353
Average velocity = -3353+175/2 = -1589m/s
Momentum of the rocket just before landing (kg m/s)
Momentum = mass * velocity
Distance = 30km (30000m) Time = 6 minutes (360 seconds)
Velocity = Distance/Time Velocity = 30000/360 = 83.333..m/s
1500kg * 83.333..m/s = 125,000kg m/s
Kinetic energy of the rocket just before landing (J)
0.5 x mv2
0.5*(1500*83.333..)2 = 7812500000 J
Gravitational potential energy at start of descent (J)
GPE = mgh
1500 * 9.8 * 30000 = 441000000 J
Force exerted by the ground on the rocket (N)
F = mass*acceleration
Acceleration = change in velocity/time = Final Vel.-Initial Vel./0.04 = 175-(-3353)/0.04=84000m/s2
F = 1500*84000 = 126000000 N
Use the idea of interaction pairs to state what the force exerted by the rocket on the ground would be.
Equal and opposite ground reaction force is exerted by the ground on the rocket.
The force exerted by the rocket on the ground would be the same exerted by the ground but in the opposite direction. It would be 126000000 N.

Analysis and Evaluation
Explain why the velocity at time of impact was much higher than the average velocity
The velocity at the time of impact was higher than the average because May you help me here please

Explain why the actual gravitational potential energy will in reality be much higher than this
The GPE will be much higher in reality as there is May you also help me here please

Explain why increasing the time of impact would mean the rocket was less likely to get damaged upon impact

Force = Change in momentum/time taken for change
Increasing the time of impact (time taken for change) increases the time taken to change the momentum which reduces the forces. It will take a lot of force to damage the rocket, so reducing the force will damage the rocket less.

5. Sep 5, 2016

Velocity increases as time increases right?

Sorry i'm kind of new to all this stuff, just starting physics a-levels and tbh ive forgotten most of GCSE physics and threw away all my books so all i have is the internet and i can't seem to find the answer.

6. Sep 5, 2016

### David Lewis

I'm going to assume motors were turned off and air resistance negligible.
We know the initial velocity at time = 0 s
We know final velocity at time = 360 s - 0.04 s
We know velocity at time = 360 s
Plot those 3 points and connect the dots.

7. Sep 5, 2016

### David Lewis

0.5 * 1500 kg * (83.333 m/s)2 = 5.21 MJ
Note that all units of measure are stated, mass is not squared, the answer is rounded off to 3 significant figures, and a unit prefix (mega-) was chosen to make the number manageable.

8. Sep 6, 2016

### CWatters

Was there a diagram provided with this problem?

How was the 30km distance measured?

9. Sep 6, 2016

### Kajal Sengupta

While calculating momentum you have calculated Velocity is distance / time. This formula can be applied when the velocity is uniform and there is no acceleration. But the falling rocket has an acceleration = 9.8 m/s^2 which you have used in the first part of the calculation.You have to make changes there.

10. Sep 6, 2016

### BvU

There is still a lot of information missing. Usually a test flight with a rocket has at least two phases: a launch phase where the thing is propelled up until the fuel is exhausted, and a free fall phase that can end with a crash or go over into a slower descent with a parachute. The launch can be vertical (did the thing reach a height of 30 km ? 15 km ?) or at an angle (did it travel 30 km horizontally ?). Your GPE calculation suggests it went 30 km high, but: did it ?

You are clearly informed that the velocity at time of impact is 175 m/s. Why calculate momentum and kinetic energy "just before landing" on the basis of an average velocity of 83 m/s ? If the thing is really shot straight up the average velocity is zero: after these 6 minutes it's back where it started !

The way I described the usual test rocket flight needs one more point (when the fuel is spent). Sure makes a difference .

11. Sep 6, 2016

### David Lewis

Thanks. Point well taken. For simplicity I assumed motors were turned off during the portion of the flight being analyzed. Whether my assumption is valid perhaps OP can confirm or deny for us.

12. Sep 7, 2016

Sorry, ill get back to this thread next week and reply, but thanks for all the help, just getting a lot of physics homework recently so this task is not due until next wekk
I appreciate all the help.

And there is no diagram + ill fix the mistakes

13. Sep 7, 2016

### CWatters

Without more info I think it's very difficult to answer the questions. For example you state that the time for the journey is 6 mins. So is that the time for the whole flight (launch to landing) or just the descent phase? You appear to have assumed the latter.

14. Sep 18, 2016

All the information I've given is for the descent of the rocket, the descent took 6minutes, sorry if it wasn't clear

15. Sep 18, 2016

### CWatters

And the "distance travelled" is that horizontal, vertical, or ?

16. Sep 18, 2016