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07 Feb 2021, 05:26
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E
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Difficulty:
25%
(medium)
Question Stats:
77% (01:53) correct
23%
(01:48)
wrong
based on 166
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Suppose the height h of an object thrown straight upward from the ground is given by h = 39.2t − 4.9t^2 where h is in meters and t is the elapsed time in seconds. After how many seconds does the object strike the ground?
A. 2 seconds
B. 3 seconds
C. 4 seconds
D. 6 seconds
E. 8 seconds
A. 2 seconds
B. 3 seconds
C. 4 seconds
D. 6 seconds
E. 8 seconds
07 Feb 2021, 05:50
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Given h = 39.2t − 4.9t^2
on factorizing, we get
h = (8 - t)*(4.9t)
so h = 0 at two instants
From the equation, (i) 8-t =0 or (ii) 4.9t = 0
By logic, once at the beginning, that is 4.9t = 0 => t=0 and
again when the ball hits the ground again which will naturally be the other case, that is, (8-t) = 0 => t = 8
hence E
on factorizing, we get
h = (8 - t)*(4.9t)
so h = 0 at two instants
From the equation, (i) 8-t =0 or (ii) 4.9t = 0
By logic, once at the beginning, that is 4.9t = 0 => t=0 and
again when the ball hits the ground again which will naturally be the other case, that is, (8-t) = 0 => t = 8
hence E
shubhamjha22
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07 Feb 2021, 07:42
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Distance one way
h= 39.2t − 4.9t^2
time take is t
Speed= Distance/Time=> h/t= 39.2 − 4.9t.
For the ball to drop back to ground
Distance Traveled= 2h
Speed = 39.2-4.9t
time= Distance/speed=> 2*(39.2t − 4.9t^2)/ 39.2-4.9t= Time(t)
t= 2*t (39.2-4.9t)/(39.2-4.9t)
1= 2* (39.2-4.9t)/(39.2-4.9t)
(39.2-4.9t)= 2*(39.2-4.9t)
(39.2-4.9t)= 0
39.2=4.9t
t= 8
Option E
h= 39.2t − 4.9t^2
time take is t
Speed= Distance/Time=> h/t= 39.2 − 4.9t.
For the ball to drop back to ground
Distance Traveled= 2h
Speed = 39.2-4.9t
time= Distance/speed=> 2*(39.2t − 4.9t^2)/ 39.2-4.9t= Time(t)
t= 2*t (39.2-4.9t)/(39.2-4.9t)
1= 2* (39.2-4.9t)/(39.2-4.9t)
(39.2-4.9t)= 2*(39.2-4.9t)
(39.2-4.9t)= 0
39.2=4.9t
t= 8
Option E
