What is the weight of the sled?

[bennyq]

Homework Statement

Simple question, if a dog jumps on a stationary sled at velocity v.. the dog weighs 20kg.
The velocity after is v/2. What is the weight of the sled

Homework Equations

m1v = (m1 + m2)v/2

The Attempt at a Solution

I rearrange this equation and i get m2 = m1.. this is fine.. But what if i use kinetic energy equations, so 1/2mv^2 before should = 1/2mv^2? In this case i get a different answer , i get m2 = 3m1?

 

[wukunlin]

What is the definition of inelastic collision?

 

[bennyq]

momentum of an isolated system is conserved, so total momentum before equals total momentum after.. for perfect inelastic collision. hence my first equation. Though i also thought no kinetic energy is lost, so kinetic energy before should equal kinetic energy after the collision?

 

[Prashasti]

In a perfectly inelastic and in an inelastic collision, the total kinetic energy is NOT conserved. Energy is dissipated in the form of heat, sound. So, the total kinetic energy before collision is NOT EQUAL to the total kinetic energy after collision.

 

[HallsofIvy]

momentum of an isolated system is conserved, so total momentum before equals total momentum after.. for perfect inelastic collision.

If you intended this as an answer to the question "what is a "perfectly inelastic collision", you are wrong. Total momentum is conserved in any situation where there is no external force.

hence my first equation. Though i also thought no kinetic energy is lost, so kinetic energy before should equal kinetic energy after the collision?

No, kinetic energy is conserved in a perfectly elastic collision, not in a perfectly inelastic collision. In a perfectly inelastic collision, the two bodies, after the collision, move together with the same velocity.

 

[Naz93]

As the others have said, in an inelatic collision, there is a maximal loss of kinetic energy (it is 0 in the centre of momentum frame). This corresponds to the two masses moving together in any other (inertial) frame. So there is only one velocity to consider in the final state, reducing the number of variables in the problem. Then momentum conservation should be sufficient to find the answer.