# Homework Help: Rocket upward thrust force

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1. Oct 20, 2015

### PhysicsStudent1

1. The problem statement, all variables and given/known data
A 590 kg rocket is at rest on the launch pad. What upward thrust force is needed to accelerate the rocket uniformly to an upward speed of 28 m/s in 3.3 s?

2. Relevant equations
V = Vi + at
F = ma

3. The attempt at a solution
I'm just confused about what "thrust force" actually means. Is this the same as normal force? When I solve for a using the first equation, a = 8.485.
Using the next equation, I get that F = 5006 N. Is this the upward force? Or do I need to solve for gravitational force (F=mg=5782) and then add 5006 to 5782 and get a total of 10,788 N?

If you could please clarify this for me that would be great! Thanks!

2. Oct 20, 2015

### SteamKing

Staff Emeritus
Draw a free body diagram of the rocket. Be careful to determine the directions in which the various forces acting on the rocket are pointing.

Thrust force is produced by the rocket motor. It's what propels the rocket upward from the earth.

3. Oct 20, 2015

### Mister T

As the rocket sits on the launch pad there is a normal force, which acts upward. The thrust force also acts upward. In both cases they are forces exerted on the rocket. The difference is that the normal force is exerted on the rocket by the launch pad whereas the thrust force is exerted on the rocket by the exhaust gases.

Mass times acceleration always gives you the net force.

4. Oct 20, 2015

### PhysicsStudent1

So I drew the FBD of the rocket and I have one arrow going upward for the normal force and one arrow going downward for the gravitational force. I haven't learned any other forces besides frictional force, so I am still not quite understanding exactly what you mean by the rocket motor producing the thrust force.

Is net force the same as normal force - gravitational force? How do I just get the thrust force?

And I apologize if I am mixing up the names of the different forces (especially normal and thrust). I thought they were the same thing

5. Oct 20, 2015

### SteamKing

Staff Emeritus
Well, the act of expelling the hot gasses out of the rocket motor creates a reaction in the rocket, which propels it in the opposite direction. How do you think space rockets lift off from the launching pad?

Normal force exists only for something sitting on the ground. It's a reaction to the gravitational force trying to pull a body to the center of the earth. When something is no longer sitting on the ground, what is the normal force then?

6. Oct 20, 2015

### Mister T

The net force is the vector sum of all the forces acting on the rocket.

7. Oct 20, 2015

### PhysicsStudent1

The thrust force? But my question is how do i calculate that? I have only learned formulas for frictional, gravitational, and normal force. I don't understand "thrust" force. Can't I just use F=ma? If not, what do I do?

8. Oct 20, 2015

### Mister T

The reaction to the normal force exerted on the rocket by the ground is the force exerted on the ground by the rocket.

The reaction to planet Earth pulling down on the rocket is the rocket pulling up on planet Earth.

9. Oct 20, 2015

### Mister T

You calculated that the gravitational force is 5782 N. And you calculated that the net force is 5006 N.

So, if you have a downward force of 5782 N, how much upward force do you need to get a net force (vector sum of forces) equal to 5006 N directed upwards?

10. Oct 20, 2015

### PhysicsStudent1

(I'm using negative to indicate the downward direction and x=upward force)
-5782 + x = 5006
x = 10,788 N

Is this the final answer? (and also thank you for taking the time to explain to me what net/thrust/normal force all mean)

11. Oct 20, 2015

### Mister T

Nice! And you're welcome. (By the way, I didn't go back and check all your work. You've got the physics right, so as long as you did the arithmetic right, you've got it.)