- #1

Konner54

- 3

- 0

- Homework Statement
- A 20kg Cart is moving horizontally along a friction-less track with a speed of 4 m/s when a 10 kg bag of sand is quickly placed on it. What is its new speed?

- Relevant Equations
- 1/2mv^2 and P=mv

Hi Folks,

This is a super simple physics problem that I just cannot understand when and where to implement the difference. Here is the problem:

A 20kg Cart is moving horizontally along a friction-less track with a speed of 4 m/s when a 10 kg bag of sand is quickly placed on it. What is its new speed?

I attempted solving this by assuming that the kinetic energy before and after must be the same, thus:

1/2 Mi Vi^2 = 1/2 Mf Vf^2. Solving for the final velocity I got 3.26 m/s

However, the correct answer is ¨¨¨~2.7 m/s, derived by using the conservation of momentum:

MiVi=MfVf. Solving for the final velocity.

Shouldn't these values be the same in a completely elastic collision such as this? What am I missing?

Any insight on when to deploy KE and when to deploy conservation of P would be great.

Thanks!

This is a super simple physics problem that I just cannot understand when and where to implement the difference. Here is the problem:

A 20kg Cart is moving horizontally along a friction-less track with a speed of 4 m/s when a 10 kg bag of sand is quickly placed on it. What is its new speed?

I attempted solving this by assuming that the kinetic energy before and after must be the same, thus:

1/2 Mi Vi^2 = 1/2 Mf Vf^2. Solving for the final velocity I got 3.26 m/s

However, the correct answer is ¨¨¨~2.7 m/s, derived by using the conservation of momentum:

MiVi=MfVf. Solving for the final velocity.

Shouldn't these values be the same in a completely elastic collision such as this? What am I missing?

Any insight on when to deploy KE and when to deploy conservation of P would be great.

Thanks!