Answer: (a)
Explanation:
Given
Mass of large truck is 4M with velocity V
Mass of small car is M which is at rest
After the collision car and truck stick together
Conserving momentum
For elastic collision, the velocity of the center of mass remains the same as the momentum is conserved and there is no acceleration involved.
So, for elastic collision velocity 
What is the unit c denotes here
Inside the torch, they usually use copper or brass to conduct electricity.
Nuclear size are measured in a unit called fermi
Magnetism at the centre of a bar magnet is zero, because there is no magnetism at all in the centre
Hope this helps :)
Explanation:
When a bullet is fired from a rifle, the actual firing makes a distinctive sound, but people at a distance may hear a second because the bullet creates a shock wave , a little sonic boom( speed of this sonic boom is greater than the speed of sound). It is this sound only that is heard by the people at a distance from the gun.
