Physics

A 2kg mass moving with a velocity of 7 m/s collides elastically with a 4 kg mass moving in the opposite direction at 4 m/s. The 2 kg mass reverses direction after the collision and has a new velocity of 3 m/s. What is the new velocity of the 4 kg mass?

My first approach was with conservation of kinetic energy.
Then I got the correct answer using the conservation of momentum formula.

However, why do I not get this same answer using conservation of kinetic energy? Both kinetic energy and momentum are conserved in an elastic collision...I'm accounting for direction changes in the conservation of kinetic energy formula as well...

And since apparently I cannot use conservation of kinetic energy for this, when DO I use conservation of kinetic energy?

  1. 👍
  2. 👎
  3. 👁
  1. The problem is fake. It is not an elastic collision. You can prove that by not assigning a velociy for after the collision, then solving for it using both equations.

    conservation of momentum holds for elastic AND inelastic equations.

    1. 👍
    2. 👎
    👤
    bobpursley
  2. Anyway, ALWAYS use momentum first. You can not count on the energy being the same before and after unless the collision is elastic. I suspect that the claim of elasticity was a typo.

    1. 👍
    2. 👎

Respond to this Question

First Name

Your Response

Similar Questions

  1. physics

    A baseball of mass m1 = 0.32 kg is thrown at a concrete block m2 = 6.25 kg. The block has a coefficient of static friction of μs = 0.71 between it and the floor. The ball is in contact with the block for t = 0.195 s while it

  2. Physics

    A white billiard ball with mass mw = 1.43 kg is moving directly to the right with a speed of v = 3.4 m/s and collides elastically with a black billiard ball with the same mass mb = 1.43 kg that is initially at rest. The two

  3. physics

    A softball of mass 0.220 kg that is moving with a speed of 6.5 m/s (in the positive direction) collides head-on and elastically with another ball initially at rest. Afterward it is found that the incoming ball has bounced backward

  4. physics

    on a air track, a 400g glider moving to the right at 2.00m/s collides elastically with a 500g glider moving in the opposite direction at 3.00 m/s. Find the velocity of the first glider after the collision Find the velocity of the

  1. Physics

    A body of mass 2kg moving with velocity of 5m/s collides with a stationary body of mas 500g. if the two bodies move together after impact. calcalate the magnitude of their common velocity

  2. physics

    a 2kg car moving towards the right at 4 m/s collides head on with a 8 kg car moving towards the left at 2 m/s and they stick together. After the collision, the velocity of the combined boides is

  3. science

    A body of mass 20kg moving with a velocity of 8m/s collides with a stationary body of mass 10kg and the two stick together.They then move with a common velocity V.Find the value of V.

  4. Elastic Collisions-Physics

    A 0.16 kg billiard ball moving at 1.20 m/s collides elastically with one of the same mass at rest. It continues at a speed of 0.6 m/s afterward in a different direction from before. What is the final speed of the other billiard

  1. Science

    An object having mass 2kg is moving with contant velocity of 5ms-1how much force is acting on the body

  2. Physics

    A proton, moving with a velocity of vi, collides elastically with another proton that is initially at rest. Assuming that after the collision the speed of the initially moving proton is 2.90 times the speed of the proton initially

  3. I need help with Physics

    A baseball of mass m1 = 0.46 kg is thrown at a concrete block m2 = 8.25 kg. The block has a coefficient of static friction of μs = 0.81 between it and the floor. The ball is in contact with the block for t = 0.165 s while it

  4. physics

    calculate the kinetic energy of a body of mass 2kg moving with a velocity of 0.1m/s.

You can view more similar questions or ask a new question.