1. Jul 30, 2011

### amochang

I'm a newbie to this subject, so forgive me if what I'm asking supposed to be common sense.

An airplane travels 280 m down the runway before taking off. If it starts from rest, moves with a constant acceleration, and becomes airborne in 8.00 s, what is its speed in m/s, when it takes off?

So, I think what is given is d=280m then t=8.00s and it is implied that it's initial velocity is 0. And what is asked is the speed or velocity...

Here's the thing. My friend keeps telling me that I should find first acceleration before speed, but I keep saying that I can simply just use d/s. Then, that conversation left me confused.

So, what formula should I really use?

2. Jul 30, 2011

### hubert_g

You should think about how distance traveled by a plane and it's speed change with time when acceleration is constant. Then you'll be able to get answer from simultaneous equations.

3. Jul 30, 2011

### Staff: Mentor

d/s will give you the average velocity over the displacement s. It won't tell you the final velocity. However, for constant acceleration there is a relationship between the average velocity and the final velocity after a given distance. Your text book or class notes should have it.

4. Jul 30, 2011

### PeterO

it is possible to solve this problem by first finding the acceleration - as your friend says. Finding the acceleration is like a half-way point.

It is also possible to use d/s - but that also is a halfway point in the calculation, not the final answer.

You could even draw a velocity-time graph, and get the answer from that.