M1

Anila is practising catching tennis balls. She uses a mobile computer-controlled machine which fires tennis balls vertically upwards from a height of 2.5 metres above the ground. Once it has fired a ball, the machine is programmed to move position rapidly to allow Anila time to get into a suitable position to catch the ball.
The machine fires a ball at 24 m s-1 vertically upwards and Anila catches the ball just before it touches the ground

Calculate length of time in the air

So I started off with this

V =u+at
V=25,u= 24,a=9.8
But apparently u is -24 why is that ..

Reply 1

Notnek

Original post by nighthawk123

Gravity acts downwards but the balls are fired upwards. But your u and a have the same sign...

u could be 24 or -24 depending on which direction you take as positive. You need to make sure that all the variables have the correct sign once you have chosen the direction.