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# Mechanics 1 HELP - Constant Acceleration Watch

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1. I'm having some difficulty with this question as it involved SUVAT, but i can only find 2 of the 4 variables i need to solve the equations.

Nathan hits a tennis ball straight up into the air from a height of 1.25m above the ground. The ball hits the ground after 2.5s Assuming g=10m/s2, find

i)The speed nathan hits the ball
ii)The greatest height above the ground reached by the ball
iii)The speed the ball hits the ground
iv)How high the ball bounces if it loses 0.2 of its speed on hitting the ground

For part i i assumed that v was 0 a was -10 and t was 2.5 which gave me an asnwer of 25 which was wrong.

Mostly need help with Part i
2. The ball doesn't come necessarily to an immediate halt when it touches the ground so you can't assume that v=0.

You know the displacement is -1.25m, the time is 2.5s, the acceleration is -10m/s^2.

An appropriate equation is
3. I am fairly sure that in this example u=0 as well
4. (Original post by Jackster45)
I am fairly sure that in this example u=0 as well
I think u is the speed he throws it at
5. (Original post by Kvothe the Arcane)
The ball doesn't come necessarily to an immediate halt when it touches the ground so you can't assume that v=0.

You know the displacement is -1.25m, the time is 2.5s, the acceleration is -10m/s^2.

An appropriate equation is
Would s not have to account the distance it goes up and down
6. (Original post by Himtiaz)
Would s not have to account the distance it goes up and down
Nope. It's the distance of the final location in relation to where you're measuring from (in this case the initial position).
7. (Original post by Kvothe the Arcane)
Nope. It's the distance of the final location in relation to where you're measuring from (in this case the initial position).
(Original post by Jackster45)
I am fairly sure that in this example u=0 as well
8. i got the greatest height as 8.45 and the speed it reaches the ground as 13
9. I got 5.51 but the book says 5.41

What i did was multiply the displacement by 0.8 and subtracted 1.25
10. I did 0.8 *8.45 -1.25 which gives 5.51 but the book says 5.41

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Updated: October 9, 2016
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