# Homework Help: Forces and Newton's Laws

1. Jul 29, 2009

### Cheddar

1. The problem statement, all variables and given/known data
A large gun accelerates a 5.0 kg mass from rest to a speed of 4000 m/s into orbit. The net force accelerating the bullet is 490,000 N.
How long will it take the projectile to come up to speed?
It doesn't say anything above air resistance or gravity.

2. Relevant equations
acceleration = net force / mass
time = velocity / acceleration

3. The attempt at a solution
0.041 seconds (using the equations listed above)

2. Jul 30, 2009

### tiny-tim

Hi Cheddar!

This question doesn't make any sense …

if the mass goes into orbit, the orbit obviously passes through the end of the gun …

in other words, if the gun is on Earth, then the mass crashes, and doesn't go into orbit.

3. Jul 30, 2009

### Kaimyn

I think I might have heard something along these lines. Does it have anything to do with this?

Good ol' Newton

Perhaps when it says large, it really means large

4. Jul 30, 2009

### Redbelly98

Staff Emeritus
Looks good, I think the problem is just this simple ... though tiny-tim is correct, since the gun is presumably on the surface of the Earth it will return to Earth after one single orbit (neglecting air resistance).

A reasonable question. Try comparing the force of gravity to the 490,000 N force being applied.

5. Jul 30, 2009

### tiny-tim

ah! thanks Kaimyn! … the ol' do-it-from-the-top-of-Mount-Everest trick!!

in that case, Cheddar, you can forget about gravity …

the gun, and the acceleration, will be horizontal (and the mass will be supported by the bottom of the barrel of the gun ).

6. Jul 30, 2009

### Redbelly98

Staff Emeritus
The acceleration occurs over roughly 80 meters, very small compared to the size of the Earth.

But the orbit issue really is irrelevant here.

7. Jul 30, 2009

### The_Doctor

4000/(490000/5) What they're basically asking is how long will it take to reach 4000m/s.

8. Jul 30, 2009

### Cheddar

Thank you everyone for your input.
Now, I realize that the fact that the mass is being projected into orbit doesn't matter.
Since the speed of the mass is going to reach 4000 m/s well before reaching orbit.
The question is whether my answer correct as it is (neglecting gravity) or should I be accounting for gravity at some point in the equations?

9. Jul 30, 2009

### Redbelly98

Staff Emeritus
I repeat: