Understanding the concept of energy

A ball is thrown vertically downwards from a height of 20m with an initial velocity . It collides with the ground, losses 50% of its energy in a collision, and rebounds to the same height. The initial velocity is__. Take  

Consider a drop of rainwater having a mass of 1g falling from a height of 1 km. It hits the ground at a speed of 50 m/s. Take . The work done by the gravitational force is__

A block of mass 10 kg moving in the X-direction with a constant speed of 10 m/s is subjected to a retarding force during its travel from x=20m to 30m. Its final KE will be__.

A man weighing 70kg carries a 30kg box to the top of the building 20m high. Calculate the work done by the man?

The potential energy of a system increases if work is done___

A toy rocket of mass 0.1 kg has a small quantity of fuel of mass 0.02 kg which it burns out in 3s. Starting from rest on a horizontal smooth track, it gets a speed of 20 m/s after the fuel is burnt out. The approximate thrust on the rocket is__

A mass of 0.5 kg moving with a speed of 1.5 m/s on a horizontal smooth surface, collides with a nearly weightless spring of force constant, k= 50 N/m. The maximum compression of the spring would be__

A ball moving with a velocity of 2 m/s collides head-on with another stationary ball of double the mass. If the coefficient of restitution is 0.5, then their velocities after collision will be__

A man pulls a box on a horizontal floor through a distance of 20m with a force of 30N  applied along a rope tied to the box and making an angle of 60° with the horizontal. The work done is__

A bomb of mass 30 kg at rest explodes into two pieces of masses 18 kg and 12 kg. The velocity of 18 kg mass is 6 m/s. The kinetic energy of the other mass is__

A body of mass 2.0 kg initially at rest is moved by a horizontal force of 0.5 N on a smooth table. The work done by the force in 8s is__

A body of mass 5 kg initially at rest is subjected to a force of 20 N. The kinetic energy acquired by the body at the end of 10s is__

300 J of work is done in sliding a 2 kg block up an inclined plane of height 10 m. Work done against friction is__

An engine pumps water continuously through a hose. Water leaves with a velocity v and mass m is the mass per unit length of the water jet. What is the rate at which kinetic energy is imparted to water?

A block of mass M is attached to the lower end of a vertical spring. The spring is hung from a ceiling and has a force constant value k. The mass is released from rest with the spring initially unstretched. The maximum extension produced in the length of the spring is__