# Homework Help: Motion - V.Confused!

1. Jan 4, 2010

### Jason17

1. The problem statement, all variables and given/known data
An aircraft of mass 5.8 x 10^4 kg stands at the end of the runway ready for take-off. The engines provide a constant total thrust of 2.1 x 10^5N

i)Calculate the initial acceleration in ms-2 of the aircraft as it starts to move

ii) Calculate the length in m of runway needed for the aircraft to reach its take off speed of 85 ms-1 , assuming acceleration is constant

2. Relevant equations
Motion Equations

v = u + at
s = (u + v)/2 * t
s = ut + 1/2*at^2

3. The attempt at a solution

The answer is 3.6ms to part i) but i cant seem to get anywhere near that.
I really dont know where to start.
Do i square both numbers and square root it? I just need a guide to lead me what info i have and what info i need.

thank you

2. Jan 4, 2010

### mgb_phys

Part (i) Have you tried F=ma ?

3. Jan 4, 2010

### rock.freak667

Newton's 2nd law, F=ma

4. Jan 4, 2010

### Marshall10488

for i) use f=ma you know the force from thrust and the mass of the object so rearrange to give
a=f/m
that will give you your acceleration then substitute that into you motion equations
you have a, u and v. you want to find t
v=u+at
rearrange to give t

5. Jan 4, 2010

### Jason17

wow..
i just figured it out myself just then.

i look very stupid now :\

Ok i got part i) done but what about part ii) ?

6. Jan 4, 2010

7. Jan 4, 2010

### Jason17

Ok so i got these

a: 3.6ms
u: 85ms
v: 0ms?
t: ?

Is that the right layout?
But why would you want to find 't'? when your asked to find the 'length' ?
I assume you get the value for 't' and substitute into s=ut+0.5at^2?

Ps. How do you distinguish something as an initial velocity(u) and final velocity(v)?
Eg. The take off speed of 85ms in the question.

Last edited: Jan 4, 2010
8. Jan 4, 2010

### mgb_phys

There is another of the motion equations with V,U,a and distance.