2 Turtle doves
Turt the turtle dove vertically down into the water at a speed of 10 meters per second. If she decelerates at 2 meters per second how far beneath the surface does Turt reach (in meters)?
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Initial velocity = 1 0 m s − 1
Final velocity = 0 m s − 1
Acceleration = − 2 m s − 2
$$v^2 - u^2 = 2as$$ $$0^2 - 10^2 = 2(-2)s$$ $$-100 = -4s$$ $$s = 25$$
We have initial velocity ( u ) = 1 0 m / s and acceleration ( a ) = ( − 2 ) m / s 2 . Since it is said that she is decelerating, she will travel the distance(say 's' m) till her velocity becomes zero, i.e, final velocity ( v ) = 0 .
We now use here the motion equation v 2 = u 2 + 2 a s . Putting the given values in the equation, we get---
0 2 = 1 0 2 + 2 × ( − 2 ) × s ⟹ 0 = 1 0 0 − 4 s ⟹ 4 s = 1 0 0 ⟹ s = 2 5
Thus we finally get the distance covered = s = 2 5
During the first second, the speed of the turtle is greater than 8 m/s. This means it does more than 8 meters. During the 2nd second, its speed is greater than 6 m/s, so it will go on for more than 6 meters. 8+6=14 meters, and this exclude three of the possible answers :P
BTW the speed decreases constantly, so you can take it with a media of 2 1 0 + 0 = 5 for 5 seconds, the time needed to make it stop. 5 ∗ 5 = 2 5
It takes 5 seconds for Turt to completely stop. If Turt did not decelerate he would have travelled a distance of 5s x 10m/s = 50m , but he did decelerate uniformly, thus he travelled a distance of 50m / 2 = 25m.
I'm not used to remember equations. Instead I prefer understanding.
v 2 = u 2 - 2 a s
0 = 1 0 2 - 2 × 2 × s
1 0 0 = 4 s
s = 2 5 m e t e r s
v =0 u = 10m/s a= -2 m/ sec. square
by 3rd eq of motion , v (sq) - u (sq) = 2as we get by substituting the values , s = 25m s = distance traveled by the Turt is 25m
Well, using Torricelli We have : V² = Vo²-2.a.∆s = > 0 = 100-4.∆s => ∆s = 25m.
Vt = Vo + at ---> Vt = 0 + 2t ---> 10 = 2t ---> t = 5. We know if S = Vo . t + 1/2 . a . t^2 ---> S = 0 + 1/2 . a . t^2 = 1/2 . 2 .5 ^2 = 25. Answer : 25
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You need to remember the 3 kinematics equations.
Here, initial velocity ( u ) = 1 0 m / s
acceleration ( a ) = − 2 m / s 2
final velocity ( v ) = 0
We need to find the displacement s .
So, apply the third kinematics equation:
v 2 = u 2 + 2 a s
0 = 1 0 2 − 2 × 2 × s
1 0 0 = 4 s
s = 2 5 meters
That's the answer!