Remember that moment before collision is equal to the moment after collision.

Plugging in our values,

Answer:
<em>The initial speed of the sprinter was 2.2 m/s</em>
Explanation:
<u>Constant Acceleration Motion</u>
It's a type of motion in which the velocity of an object changes by an equal amount in every equal period of time.
The following relation applies:

Where a is the constant acceleration, vo the initial speed, vf the final speed, and t the time.
The sprinter speeds up from an unknown initial speed to vf=3 m/s in t=2 seconds with an acceleration of
.
To find the initial speed, we solve the equation for vo:

Substituting the values:



The initial speed of the sprinter was 2.2 m/s
Answer:
Length of the pendulum will be 3.987 m
Explanation:
We have given time period of the pendulum T = 8 sec
Acceleration due to gravity 
We have to find the length of the simple pendulum
We know that time period of the simple pendulum is given by



So length of the pendulum will be 3.987 m
Answer:
(A) 60 J
Explanation:
At state 1
KE₁=100 J
At state 2
KE₂ = 0
U₂=80 J
Given that surface is rough so friction force will act in opposite to the direction of motion
Lets take work done by friction = Wfr
From work power energy
Work done by all forces = Change in kinetic energy
Wfr + U₂=ΔKE
Wfr+80 = 100
Wfr= 20 J
Now when book slides from top position then
Wfr+ U = KEf - KEi
-20 + 80 = KEf-0
KEf= 60 J
(A) 60 J