I tried answering this question but everything was pretty much a wild guess. So basically I separated the initial velocity into vectors and assumed that the vertical distance would be 2.1 and got either .1 and 3.8 and then I thought maybe there was a second system at the same time but .3 seconds less since it needs to stay within the 3.8 seconds? I plugged in everything after but I'm not sure if this actually works? Thanks!
Hey @jebb,
It is great to see you on the site!
Okay, firstly, you accidentally made a silly mistake and took the initial velocity to be 25 ms^-1, when it is actually 15 ms^-1. The rest is SUVAT. You have to find the time taken for the ball to reach a height of 2.1 m and use the second time as this is when it is over the net (you did this but used the wrong initial velocity).
Then, we use this time to work out the horizontal distance that the tennis ball has travelled. Subtract 10 m away from this as the opponent is 10m away from him. Finally, subtract 0.3 seconds away from the time taken. Use the time taken and distance travelled (after subtracting the relevant numbers from each of those values) to calculate the average speed.
I have attached my workings below. Let me know if you don't understand :)