Homework Help: Some mechanics questions

1. Feb 24, 2004

Auron87

We have been given a set of questions to do. Most of these I have managed fairly easily but there are some which I am really stuck on and am getting quite annoyed with now as I have been working at them for ages! Help on any of these would be really appreciated:

The Saturn V rocket which launched the Apollo space missions had the following specifications:
mass at lift-off = 3.0 x 10^6 kg
velocity of exhaust gases = 1.0 x 10^4 m/s
initial rate of fuel consumption at lift-off = 3.0 x 10^3 kg/s

1.(a) Calculate:
(i) the force (thrust) produced at lift-off
(ii) the resultant force acting on the rocket at lift-off

2.(a) An empty railway truck of mass 10000kg is travelling horizontally at a speed of 0.5m/s. (had to calculated the momentum and kinetic energy - not sure if this will help with the bit im stuck with!)
Sand falls vertically into the truck at a constant rate of 40kg/s. Calculate the additional horizonal force which must be applied to the truck if it is to maintain a steady speed of 0.5m/s.

Thanks for anyones help - I can usually manage these kind of questions but I'm really stuck for some reason!!

2. Feb 24, 2004

ShawnD

$$Ft = mv$$

Derive both sides

$$F = (\frac{dm}{dt})(v)$$

I think this means to include gravity. It should be whatever you got from part (i) - gravity.

$$Ft = mv$$

Derive both sides

$$F = (\frac{dm}{dt})(v)$$

Last edited: Feb 24, 2004
3. Feb 24, 2004

Auron87

I've not met that formula before but I do understand where its from but how can you use it because you don't know time or is it necessary to work out the time or something??

4. Feb 24, 2004

ShawnD

dm/dt is a variable, not a division of 2 or 4 variables

In the first question, dm/dt is 3.0 x 10^3 kg/s

In the second question, dm/dt is 40kg/s