A ball in thrown against a wall, it rebounds with speed unchanged. Which of the

Question

A ball in thrown against a wall, it rebounds with speed unchanged. Which of the following is true? The kinetic energy of the ball is unchanged, the momentum of the ball is unchanged, both kinetic energy and momentum are unchanged, both kinetic energy and momentum changed. In order to have work; there must be a net force, there must be motion, both (a) and (b). The work done by a force perpendicular to the displacement of the object is maximum. positive. zero, done of these. As a skier coasts down the incline; a) the kinetic energy is being converted to potential energy, b) potential energy is being converted to kinetic energy, c) kinetic and potential energies remain the same. d) all of these. An object is falling freely under gravity. At any point during its fall: a) potential energy is conserved, b) kinetic energy is conserved, c) kinetic energy and potential is conserved, d) kinetic energy plus potential energy conserved. A hockey puck slides along the horizontal floor to the right. Which of the following is not true; the puck is slowing, the net work done on the puck is negative, the work done by gravity is negative, the puck will not continue to move indefinitely. The kinetic energy of a moving mass is proportional to the; velocity, square of the velocity, squareroot of the velocity, none of these. An electron is moving right with a speed 'v' and kinetic energy 'K'. Later on the same electron is moving left with a speed '2v'. What is it's kinetic energy? 2K, - 2K, 4K, -4K.

Explanation / Answer

(5) . Initial momentum = mv

Final momentum = m(-v) Since it rebound its velocity direction is changed.

So, Momentum is changed .Since magnitude of momentums are equal but the direction are opposite.

Initial kinetic energy K = (1/2)mv 2

Final kinetic energy K ' = (1/2) m(-v) 2

=  (1/2)mv 2

So, kinetic energy is unchanged.

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