Conservation of Momentum/Energy

[Shark 774]

Two roller-bladers are skating with really good skates on a surface that has virtually no frictional resistance to their rolling. Malachi has a mass of 55kg. He rolls up to Sarah at 5m/s. Sarah has a mass of 45kg and is rolling in the same direction as Malachi at 1m/s. They join hands and roll on in the same direction. They do not skate. Which of the following describes the physics of this 'collision' best?
A The combined momentum and KE has not changed.
B The combined KE has remained constant.
C The combined momentum has remained constant.
D Both the combined momentum and KE have changed.

The answer says "C momentum is conserved in a closed system" (such descriptive answers...) but if it's a closed system then why hasn't KE remained the same as well??

Thanks guys.

[xZero]

im guessing that some of the kinetic energy has transformed into another form of energy

[Shark 774]

Stupid question... I just realised that of course energy won't be conserved because truly elastic collision only happen at atomic levels.

[SDPHD]

Stupid question... I just realised that of course energy won't be conserved because truly elastic collision only happen at atomic levels.

Sometimes, in textbook questions that contain ideal situations (ie no friction), elastic collisions do occur in real world scenarios, although, without a final velocity, it isn't possible to ascertain whether the initial and final kinetic energies are the same or different.

It's a pretty confusing question - I have no idea what 'They do not skate' means. Does it mean they come to a sudden stop? On a frictionless plane, they wouldn't gradually come to a stop either.

Confusing. :idiot2:

[Bonifacio]

Stupid question... I just realised that of course energy won't be conserved because truly elastic collision only happen at atomic levels.

Actually your wrong, elastic collisions can happen in real world scenarios (not actually but we assume they can), in an isolated system momentum is always conserved, but not necessarily kinetic energy.

'They do not skate': If they have no friction forces, than any further skating would result in an acceleration. Hence they are at constant velocity.

The way to approach this question is to firstly acknowledge that obviously it is a closed system as no external force has been mentioned, this momentum is conserved, than work out initial and final kinetic energy to check if it is an elastic collision. Then select your answer!

Hope that helps.

It's not a stupid question at all, it is a previous VCAA exam question and this what is between you and an A+ on your midyear Physics exam, therefore you adapt yourself to be able to answer that question.

[Shark 774]

Stupid question... I just realised that of course energy won't be conserved because truly elastic collision only happen at atomic levels.

Actually your wrong, elastic collisions can happen in real world scenarios (not actually but we assume they can), in an isolated system momentum is always conserved, but not necessarily kinetic energy.

'They do not skate': If they have no friction forces, than any further skating would result in an acceleration. Hence they are at constant velocity.

The way to approach this question is to firstly acknowledge that obviously it is a closed system as no external force has been mentioned, this momentum is conserved, than work out initial and final kinetic energy to check if it is an elastic collision. Then select your answer!

Hope that helps.

It's not a stupid question at all, it is a previous VCAA exam question and this what is between you and an A+ on your midyear Physics exam, therefore you adapt yourself to be able to answer that question.

I meant my question was stupid, not the actual problem.