- #1
Peter G.
- 442
- 0
Hi,
I was trying to figure whether a ball would have a larger exiting velocity if it started from rest or if it was coming towards me at a given velocity.
My first thought is that it would have a greater exiting velocity if it started at rest. When I hit an incoming ball it will change direction and, since momentum is a vector, I believed the exiting speed would be smaller.
I tried using some numbers from an old exercise question:
0.07 kg Ball
Time of Contact between Ball and Tennis Racket: 0.1 second
Maximum force exerted by Tennis Player 28 N
Incoming Speed: -10 m/s
The change in momentum the Tennis player can produce: F x Δt = 2.8 kgms
(I set the incoming velocity as negative)
2.8 = 0.07v - (0.07 * (-10))
2.8 = 0.07v + 0.7
v = 30 ms
Now, if the ball started at rest:
2.8 = 0.07v - 0
v = 40 ms
But if I set the incoming velocity as positive for example:
2.8 = (0.07 * -v) - 0.7
3.5 = (0.07 * -v)
v = - 50 ms
And this would change the whole thing!
I am probably making a stupid mistake somewhere, I am confused
Thanks,
Peter G.
I was trying to figure whether a ball would have a larger exiting velocity if it started from rest or if it was coming towards me at a given velocity.
My first thought is that it would have a greater exiting velocity if it started at rest. When I hit an incoming ball it will change direction and, since momentum is a vector, I believed the exiting speed would be smaller.
I tried using some numbers from an old exercise question:
0.07 kg Ball
Time of Contact between Ball and Tennis Racket: 0.1 second
Maximum force exerted by Tennis Player 28 N
Incoming Speed: -10 m/s
The change in momentum the Tennis player can produce: F x Δt = 2.8 kgms
(I set the incoming velocity as negative)
2.8 = 0.07v - (0.07 * (-10))
2.8 = 0.07v + 0.7
v = 30 ms
Now, if the ball started at rest:
2.8 = 0.07v - 0
v = 40 ms
But if I set the incoming velocity as positive for example:
2.8 = (0.07 * -v) - 0.7
3.5 = (0.07 * -v)
v = - 50 ms
And this would change the whole thing!
I am probably making a stupid mistake somewhere, I am confused
Thanks,
Peter G.