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# Mechanics Question-Kinematics watch

1. A tennis ball is projected vertically upwards with a speed of 6.3m/s from a platform that is 28m above the floor. At what time does it strike the floor?
I said:
v=0
u=6.3
a=-9.8
t=?
And I used v=u+at and got 0.642 seconds but I think that's wrong.
2. (Original post by Ahrimylove)
A tennis ball is projected vertically upwards with a speed of 6.3m/s from a platform that is 28m above the floor. At what time does it strike the floor?
I said:
v=0
u=6.3
a=-9.8
t=?
And I used v=u+at and got 0.642 seconds but I think that's wrong.
You don't know that the final velocity is 0. Try drawing a diagram, a ball is thrown from a platform 28m above the floor, and you are working out the time for when it hits the floor, does this tell you anything about the displacement?
3. (Original post by Ahrimylove)
A tennis ball is projected vertically upwards with a speed of 6.3m/s from a platform that is 28m above the floor. At what time does it strike the floor?
I said:
v=0
u=6.3
a=-9.8
t=?
And I used v=u+at and got 0.642 seconds but I think that's wrong.
Your right that your wrong. What you calculate is fir the upward motion only
4. If you set (taking upwards as positive)
S=-28m since the vertical displacement when the ball hits the ground will be -28m because we are taking upwards as positive
u=6.3
v=V
a=-9.8
t=T
Now just set up an equation with time as one of the variables and solve for T
This will lead to solving a quadratic equation. You will get 2 answers and you have to reject one and it should be obvious which one to reject.
5. (Original post by B_9710)
If you set (taking upwards as positive)
S=-28m since the vertical displacement when the ball hits the ground will be -28m because we are taking upwards as positive
u=6.3
v=V
a=-9.8
t=T
Now just set up an equation with time as one of the variables and solve for T
This will lead to solving a quadratic equation. You will get 2 answers and you have to reject one and it should be obvious which one to reject.
Okay so I formed a quadratic equation using s=ut+1/2at^2 and my equation was 4.9t^2-6.3t-28=0. I solved this and got 2.95 as my positive answer.
6. (Original post by Ahrimylove)
Okay so I formed a quadratic equation using s=ut+1/2at^2 and my equation was 4.9t^2-6.3t-28=0. I solved this and got 2.95 as my positive answer.
I haven't done it but I will do it when I get back (I'm out) and will tell you if you're right.
7. (Original post by Ahrimylove)
Okay so I formed a quadratic equation using s=ut+1/2at^2 and my equation was 4.9t^2-6.3t-28=0. I solved this and got 2.95 as my positive answer.
I got 3.12s as the answer but I formed the same quadratic as you so you mus that've made a mistake in solving the quadratic that you given.
8. Got same as B_9710

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