answer for these two questions If a car rolls gently (initial velocity-D) off a

Question

answer for these two questions

If a car rolls gently (initial velocity-D) off a vertical cliff, how long does it take it to reach 85 km/h? None of the answers below 12s 1 24 2 A stone is hrown vertically upwards with a speed of 18.0 m/s. (a) How fast is it moving when it reaches a height of 11.0 m? (b) How long is required to reach this height? (c) Why are there two answers to (b)? O (a) + 10.4 m/s ib)t 2.90 s, D.775s) Should have one time anly iD a +/-10.4 m/s ib) t: 2.90 s and D.775 s ic) one tme on the way up (D.775s) and on on the wsy down (2.905) None of the ahove

Explanation / Answer

1) initial speed = 0 m/s

final speed = 85 km/hr = 23.61 m/s

acceleration=acceleration due to gravity=9.8 m/s2

time taken to reach 23.61 m/s = 23.61/9.8=2.4 seconds

third option is correct

2) a)initial speed = 18 m/s

height reached = 11 m

let speed after reaching height of 11 m be v

v2=u2+2ah

a=-9.8 m/s2

u=18 m/s

h=11 m/s

v = 10.4 m/s

b)time required to reach thiis height = (18-10.4)/9.8=0.77 second

also, after the stone reached maximum height, it will fall and reach height of 11 m again

time required to reach maximum height = 18/9.8=1.837 second

maximum height reached =182/2*9.8=16.53 m

16.53-11=5.53 m

time required by stone to fall through a height of 5.53 m after reaching maximum height = sqrt(2*5.53/9.8)=1.06 second

1.06+1.837=2.9 seconds

Therefore, the stone reached a height of 11 m after 0.77 second and 2.9 seconds

second option is correct

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