HELP! 2 different answers with SUVAT?

I was working through the physics A-Level course on Uplearn when I came to the question

"A bullet is fired at a speed of 940ms^-1 in the horizontal plane, with an initial vertical speed of 0ms^-1. The gun is fired from a raised platform. The bullet travels 300m in the horizontal plane before hitting the ground. Calculate the height of the platform. You may ignore air resistance."

I recognized this as a double SUVAT question and knew I had to work out the time so I can get enough components for the vertical motion. So I end up with:

horizontal:
s = 300
u = 940
v = 0
a = ?
t = ?

I then proceed to work out t using s=1/2(u+v)t
rearranged to t = S/(0.5*(u+v))
so: 300/(0.5*(940+0)) = 0.64

now, I ended up with what uplearn said was the wrong answer after using s=ut+1/2at^2 for the vertical and getting 2.0m

Uplearn gives the model answer as:

"
> We are concerned with calculating the height of the platform, which is equal to the final vertical displacement of the bullet.

> In the vertical plane, we only know that the initial velocity is 0 (u=0), and that the acceleration is -9.81ms^2

> Therefore we don't have enough information to calculate the displacement

> However we can find the time of flight by considering the horizontal components

> We can use s = ut + (at2)/2 => as a=0 in the horizontal, this becomes s = ut. Therefore time of flight = t = s/u = 300/940 = 0.32 seconds.

> We then use this information in the vertical plane
..."

Now, I noticed that their answer for t is exactly half of mine and it seems that this is because the 1/2 in their method disappears because 0.5*0 = 0.

Now, the only thing I can think of that might be an issue here is that they have assumed the horizontal acceleration is 0.

Can you assume acceleration is 0 in this situation?
Which answer is actually right? Please help, thanks in advance :smile:

Reply 1

TheConfusedMedic

Yes you assume that horizontal acceleration is 0 because you are given in the question that you can ignore air resistance, and so there are no horizontal forces acting on the bullet.
So the answer for t is 0.32 seconds.

Reply 2

Spatchy

Original post by surina16

ok, thanks, just to clear things up, what did I do wrong in my answer? As far as I can tell the way I've done it should also work, so unless I just broke the SUVAT equations, I've gone wrong somewhere lol

Reply 3

TheConfusedMedic

Original post by Spatchy

I believe that it is because you have assumed that the horizontal velocity is 0 when the bullet hits the ground. When looking at projectiles questions, if the question states that there is no air resistance in the horizontal plane, I think (unless stated otherwise) that a is always 0, meaning u = v. So in this question v = 940, not 0.

Reply 4

Spatchy

Original post by surina16

ahh ok, that makes sense! I find it funny that I accused them for assuming a=0 when in fact, I had assumed v=0 in a different part of the question!
thanks for the help.

(edited 6 years ago)

Haha no problem! :h:

Hi, i noticed u were using Uplearn! I’ve been considering getting it, is it any good? can u get help from tutors relatively quickly? btw im in year 12 and i want to get it for maths :smile:

Reply 7

Spatchy

Original post by Kitty_chin

Hi, i noticed u were using Uplearn! I’ve been considering getting it, is it any good? can u get help from tutors relatively quickly? btw im in year 12 and i want to get it for maths :smile:

Sorry for my very late reply (I don't use TSR much) not sure how useful this will be to you now but I thought I would reply anyway. I used uplearn while their physics course was still in beta so it was free. I found the videos quite helpful but I'm not sure it's worth paying full price for, there are other free resources out there.