[Spain777]

I need to know how many Kgf are needed to put 70 Kg (an average person) at 40.320 Km/h. That is the speed to go out of earths atmosphere.

I know that:

1 Kgf equals 9.8 Newtons
1 Newton puts 1 Kg up to 1 m/s
11.2 Newtons equals 40.320 Km/h

Anyone can help?

[Abhimanyu Pudi]

F = ma
So, a = F/m
v = at
11.2 = F/70 (t=1)
F = 784 N = 80 kgf

[Spain777]

Dear Abhimanyu, thanks a lot for your reply.

I'm not good at all with mathematic formulas. Is it possible to explain a bit more your answer? Are you saying that I need to create a force of 80 Kg to create 784 Newtons? Would that force, speed 70 Kg up to 40.320Km/h?

Is it possible that 80Kgf gets 70Kg out of the earths gravitational field? Ins't it almost an equal weight?

Sorry, I know those are a lot of questions. It is probably simple to you, but for me this looks very complicated...

Thanks you again for your answer, hope to hear from you again.

Best to you,
Elias

[ebaines]

The basic premise of this question is wrong. It is not true that 1 newton of force moves a 1 Kg mass at a velocity of 1 m/s. From F=ma you know that 1 N causes a 1 Kg mass to accelerate at 1 m/s^2. It's a big difference. The velocity achieved depends on how long the force is applied, which you didn't tell us. But if the force is applied for one second, then the 1 Kg object will achieve a velocity of 1 m/s.

The force required to accelerate mass m to velocity v in time t is: F=mv/t. This gives F in newtons; to convert to Kgf divide by g (9.8 m/s^2). Also, this value is the net force that the object experiences, meaning the force of the engine minus the force working against it due to gravity. So if you want to determine how powerful the rocket engine must be you need to add the rocket weight to the equation. Hence the total force of the rocket engimne is



Also, 40.32 Km/Hr is NOT the speed needed to escape earth's atmosphere. Escape velocity is more like 25,000 Km/Hr.