# Homework Help: Projectile motion problem - tennis player hits a ball

1. Mar 1, 2013

### 5ymmetrica1

1. The problem statement, all variables and given/known data
A tennis player hits a ball horizontally towards a net of height 0.90 meters, at an initial speed of 25m/s. The ball is hit at a height of 2.5 meters above the ground and at a horizontal distance of 15 meters from the net. Ignore the effects of air resistance and spin

(a) show that the time it takes the ball to reach the net is 0.6 seconds
(b) hence calculate the change in height of the ball during this time
(c) determine the height at which the ball hits the net

2. Relevant equations

s = vt+1/2gt2

3. The attempt at a solution
(a) sv = vvt + 1/2 gvt2

∴t =√2sv/gv
∴t = 0.7 seconds
but t should = 0.6 seconds

(b) sv = 1/2 gvt2

∴sv = 1/2 x 9.8 x 0.62
∴sv = 1.764 meters

(c)
2.5-1.764 = 0.736 meters

but I know these answers are wrong, since part (a) is wrong. Where am I going wrong?

2. Mar 1, 2013

### mishek

Why did You use that formula?

3. Mar 1, 2013

### voko

The initial velocity is HORIZONTAL. Is the any HORIZONTAL acceleration? What equation should you use for the HORIZONTAL displacement (a)?

4. Mar 1, 2013

### 5ymmetrica1

Thanks for the reply? which formula am I supposed to use with the given data? I thought when the launch angle was 90 degrees to g that this is formula I needed to use.

Should I use vt = v0 + gt

5. Mar 1, 2013

### voko

g is is the acceleration due to gravity. It is VERTICAL. Forget about the vertical motion for a second, think only about the horizontal motion. This is what (a) is all about.

6. Mar 1, 2013

### 5ymmetrica1

ahh yes of course, I understand now so sh =vht

so t = sh/ vh

∴t = 15/22

∴t = 0.6 seconds

How are my answers for part (b) and (c)?

7. Mar 1, 2013

### voko

Now it is all fine.

8. Mar 1, 2013

thanks voko!