A-Level Physics Mechanics help - "tennis ball"

In a game of tennis, a ball is hit horizontally at a height of 1.2 m and travels a horizontal distance of 5.0 m before reaching the ground. The ball is at rest when hit.

Calculate the initial horizontal velocity given to the ball when it was hit.

So, using vertical motion for 1.2 m I found the t = 0.49 secs
Now, I used s = 0.5(u+v)t to find out v = 20 ms-1

However, in the answer is 10 ms-1

The mark scheme uses v = s/t

My question is why do we not use the suvat equation here? And therefore, just for future, when do I use suvat and when do I use v = s/t?

Thanks in advance!

Reply 1

hay.hay0311

s=0.5(u+v)t applies when there is a uniform acceleration, however because horizontal and vertical components are independent, the horizontal part of the motion does not have the effect of gravity and we neglect air resistance so there’s no acceleration to the horizontal component, therefore s=vt applies instead.

Reply 2

peristutor

The horizontal motion has no acceleration, so the velocity is constant.

Reply 3

hay.hay0311

I’m also tempted to say that almost in all cases of projectile motion questions, only s=vt is used for the horizontal motion.

Reply 4

AshKetchum1206

But there is no horizontal acceleration, i.e. a = 0 ms-2

Would that still not be classified as uniform? As it remains constant (albiet, zero, but still constant).

Reply 5

hay.hay0311

Original post by AshKetchum1206

But there is no horizontal acceleration, i.e. a = 0 ms-2
Would that still not be classified as uniform? As it remains constant (albiet, zero, but still constant).

Uniform acceleration only means the “amount” of acceleration doesn’t change (constant) during the motion, If there’s no acceleration then you don’t even need to consider if it is uniform or not since acceleration doesn’t exist in that case. Suvat equations only apply to uniform acceleration motions, not motions with no acceleration.