The discussion revolves around a physics problem involving kinetic and potential energy of a mass projected vertically. The original poster presents a scenario with a 55 kg mass projected at an initial speed of 30 m/s, seeking to calculate the original kinetic energy, the kinetic energy after 4.5 seconds, and the change in gravitational potential energy over that time period.
The discussion is active, with participants exploring various approaches to the problem. Some guidance has been provided regarding the use of conservation of energy, and there is an ongoing examination of how to relate kinetic and potential energy changes without needing to calculate height directly.
Participants note that the problem does not provide height information, which raises questions about how to approach the change in potential energy. There is also a recognition that the initial and final speeds are critical for calculating kinetic energy differences.
This is correct.physics(L)10 said:a) Original =(55)(30)2/2
This is not correct. Rethink how to calculate the speed; it's not equal to gt. (It's slowing down, for one thing. The acceleration is negative.)b) W=(m/2)(gt)2
=(55/2)[(9.8)(4.5)]2
Good. (I assume you mean v2 = ..., not 0 = ...)physics(L)10 said:ok I think I know,
v2=v1 -at
0=55-(9.8)(4.5)
You need the answer to b to get c. (You aren't given the height.)And how about c? you didn't comment on that
Not exactly. Use conservation of energy like this:physics(L)10 said:Yes my bad. and for c, does it look something like this?:
mgh=mv2/2
Those would be the speeds, from which you'd calculate KE1 and KE2 and then the difference.physics(L)10 said:So let's say, initial speed = 55 and final = 25. These would be the only differences in KE1,KE2?
You do not need the height difference. They ask for the change in PE, which can be found using conservation of energy.physics(L)10 said:How would you find out the height difference to find the potential energy difference?
I think you mean -ΔKE = ΔPE. Sure that will work--that's equivalent to the equation I gave in post #6.physics(L)10 said:Would this work? I find change in KE which then can be used in the equation -KE=PE?