AP Mechanics

fizman

use the questions that follow the animation to begin the conversation.

zaggle56

The two major concepts at work here are conservation of momentum, i.e. the momentum of the small block before it hits the big block is equal to the momentum of the big block after it is hit, and conservation of Kinetic Energy.

MrMO: a little fast on the cons. of energy part. kinetic energy is not conserved. if friction exists, even mechanical (kinetic + potential) is not conserved. then, any statement of conservation must include a term for the work done by friction or heat generated by friction.


The KE of the orange block is equal to the KE of the green block minus the work done by Friction of the block on the ground.

MrMO: unless elastic, energy is not conserved in collisions!

The KE is then transfered to the spring where it is stored and and eventually released (the work stored in the spring is not equal to the work done by the block on the spring because of the work done to it by friction).

MrMO: yes it does! look at it like this. the kinetic energy of the orange block is converted to potential in the spring and the heat generated by friction. a better statement would be: all of the blocks kenetic energy is not stored, some is turned to heat by the work done by friction.


If there were no friction then all of the energy in the system would be conserved throughout. The KE of the green block would equal the KE of the Green block. No matter what, the momentum would always be conserved. :)

MrMO: again, you are assuming an elastic collision. remember, heat is generated in two ways: friction and collisions.

zaggle56

Also, its 3 not 4.

fizman

sometime it takes a while for the server to process the posts. so far i haven't had a post not show. assume your post worked even though it does't show right away.

fizman

hey zag, i made some comments inside one of your many clones above.

jeffiziks

I cant tell who said what in that long post...and zaggle said pretty much everything in that book, but here it goes...

The KE of the orange box after the collision, elastic potential of the spring, and work done by friction will all add up to equal the original KE of the green box.

MrMO: Absolutely not! what about the collision? if a collision is not "elastic", mechanical energy is not conserved!

If friction was not present, then the mechanical energy would be conserved...

MrMO: not! the collision again. energy is converted to heat in non-elastic collisions.

saveferris

1. the two major concepts are conservation of momentum and conservation of energy.

2. im guessing the two blocks have the same mass because it looks like the green block transfers all of its momentum to the orange block.

MrMO: yes, momentum is always conserved in collisions. nothing was mentioned about mass nor type of collision. if the collision were eleastic, it would have been stated.

in that case, the green blocks KE immediately before the collision is the total energy in the system and is equal to the orange blocks KE after the collision plus the work done by friction and the work being done on the spring.

3. yes. if the system is frictionless, the KE of the orange block plus the work its done on the spring will be equal to the green blocks KE before the collision

MrMO: what about heat lost during the collision?

jeffiziks

gravity...

chelsealane

I guess the graphics and problem were taken off... I really need to learn to reply to these on time. DANG.

fizman

sorry, i'm having server problems. check back tomorrow.

jeffiziks

This is big German speaking...through Jiffkins password, I dont see the problem but I'm gonna go with F=ma and that everything else is negligiable.

jeffiziks

gravity...

6strikekickball

i don't see the graphic here and problem six just says code/code/code/