please someone help!!!!!!!! 10 points best answer?
To kick a goal in rugby you must kick the ball over the crossbar of the goal posts (height 3.0m) between two uprights. Mark attempts to kick from a distance of 35m. The initial velocity of the ball is 20ms-1 at 30 ° to the horizontal. The ball is aimed between the uprights and no spin is applied.
(i) how long does it take for the ball to reach the goal posts?
(ii) does is go over the cross bar
(iii) Later in the game mark takes another kick from the same position at 30 ° to the horizontal , find the initial speed
ANSWERS = 8(i) 2.02s
(ii) No height 0.2m
(iii) 21.57ms-1
PLEASE SOMEONE HELP WITH WORKING OUT im really struggling with these atm
hi , I answer you with pleasure:
1- the Hz. distance x =35 m
& x = Vx * t
35 = 20 m/s * Cos(30) * t
So t= 35/(20*Cos(30)) = 35/17.32051 = 2.0207 seconds
answer is t = 2.0207 sec
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2- y = Voy * t1 – 1/2 * g * t1 ^2
t1 is the time of flight to the Max. height which is = t / 2 = 1.01 sec
So y = 20 * Sin(30) * 1.01 – 0.5 * 9.81 m/sec2 * 1.01^2
y= 10.1 – = 5 m
or use Vf^2 = Vi^2 + 2 * a * s
zero = ( 20 * Sin(30) ) ^2 – 2 * g * y
0 = 100 – 2 * 9.81 * y
y = 100/ ( 2*9.81 ) = 100/19.62 =5.096 m
here answer is no the ball went above the cross bar by 2 m more height.
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3- Now the Last answer :
Set y = 3m
and use the formula : Vf^2 = Vi ^ 2 + 2 * a * y vertically
Vf = zero at 3 m height.
0 = Vy ^2 – 2 * g * y
0 = ( V * Sin(30) ) ^2 – 2 * 9.81 * 3
now find V : V ^2 * ( 0.5) ^2 = 2* 9.81 * 3
V^2 = 58.86 / (0.25) = 235.44
hence V = 15.344 m/sec
If you want V to be Your given answer 21.57 m/s the height y must be higher than 3m a little bit
Or We have not Under stood your problem well.
or Some times some books printed erroneous answers .
or check my formulas if I missed some thing .
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I hope I Served You .
RUGBY – Laws of the Tackle – Part 3
