# Kinetic energy and momentum

1. Jul 12, 2004

### wilmerena

do 2 objects that have the same kinetic energy necessarily have the same momentum? I cant think of a simple example :yuck:

2. Jul 12, 2004

### AKG

Consider a 5 kg mass going at 50 m/s, thus having 6.25 kJ of kinetic energy. Now, a mass of 125 kg going 10 m/s also has 6.25 kJ of kinetic energy. However, the first object has a momentum of 250 kg*m/s, and the second has 1250 kg*m/s of momentum.

3. Jul 12, 2004

### wilmerena

thanks so much =o) !!

4. Jul 13, 2004

### wilmerena

what if they both have 0 momentum, so does it follow that the kinetic energy of the system doesnt have to be zero as well? or does it?

5. Jul 13, 2004

### Staff: Mentor

Think about it. For a simple object:
$$KE = 1/2 mv^2$$
$$momentum = mv$$
The only way an object can have zero momentum is if its speed (v) is what? Then what is its KE?

6. Jul 13, 2004

### wilmerena

thanks again, i think i just spaced out on that one ;oP