On the second day of Christmas, Calvin gave to me

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)?

10 2 25 5

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9 solutions

Ajay Maity
Dec 22, 2013

You need to remember the 3 kinematics equations.

Here, initial velocity ( u ) (u) = 10 m / s 10m/s

acceleration ( a ) (a) = 2 m / s 2 -2m/s^{2}

final velocity ( v ) (v) = 0 0

We need to find the displacement s s .

So, apply the third kinematics equation:

v 2 = u 2 + 2 a s v^{2} = u^{2} + 2as

0 = 1 0 2 2 × 2 × s 0 = 10^{2} - 2 \times 2 \times s

100 = 4 s 100 = 4s

s = 25 s = 25 meters

That's the answer!

nice answer

sonu sekar - 7 years, 5 months ago

nice

himanshu pundir - 7 years, 5 months ago
Yan Yau Cheng
Dec 22, 2013

Initial velocity = 10 m s 1 = 10 ms^{-1}

Final velocity = 0 m s 1 = 0 ms^{-1}

Acceleration = 2 m s 2 = -2 ms^{-2}

$$v^2 - u^2 = 2as$$ $$0^2 - 10^2 = 2(-2)s$$ $$-100 = -4s$$ $$s = 25$$

Nicely solved !

Devesh Rai - 7 years, 5 months ago

Great

Ewerton Cassiano - 7 years, 5 months ago
Prasun Biswas
Dec 22, 2013

We have initial velocity ( u ) = 10 m / s (u)=10 m/s and acceleration ( a ) = ( 2 ) m / s 2 (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 (v)=0 .

We now use here the motion equation v 2 = u 2 + 2 a s v^{2}=u^{2}+2as . Putting the given values in the equation, we get---

0 2 = 1 0 2 + 2 × ( 2 ) × s 0 = 100 4 s 4 s = 100 s = 25 0^{2}=10^{2}+2\times (-2) \times s \implies 0=100-4s \implies 4s=100 \implies s=\boxed{25}

Thus we finally get the distance covered = s = 25 = s = \boxed{25}

Luca Bernardelli
Dec 22, 2013

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 10 + 0 2 = 5 \frac{10+0}{2} = 5 for 5 seconds, the time needed to make it stop. 5 5 = 25 5 * 5 = \boxed{25}

Luca Bernardelli - 7 years, 5 months ago

Log in to reply

Use Torricelli

Ewerton Cassiano - 7 years, 5 months ago
Yong JQuan
Dec 27, 2013

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.

Yong JQuan - 7 years, 5 months ago
Rahma Anggraeni
Apr 30, 2014

v 2 v^{2} = u 2 u^{2} - 2 a s 2as

0 0 = 1 0 2 10^{2} - 2 × 2 × s 2 \times 2 \times s

100 100 = 4 s 4s

s s = 25 m e t e r s \boxed{25 meters}

Puja Shree
Dec 24, 2013

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.

Budi Utomo
Dec 23, 2013

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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