Lionblaze16 Posted November 26, 2016 Share Posted November 26, 2016 In the following diagram, a force F acts on a cart in motion on a frictionless surface. The initial and final velocities of each cart are shown. Rank the energy required to change each cart's velocity from greatest to least. A: Weighs 2 kg, 5 m/s to 2 m/s - Change in KE = 1/2*2*(2*2-5*5) J = -21 J B: Weighs 3 kg, 3 m/s to -3 m/s - Change in KE = 1/2*3*(3*3-(-3)*(-3)) J = 0 J C: Weighs 5 kg, 5 m/s to 6 m/s - Change in KE = 1/2*5*(6*6-5*5) J = 27.5 J D: Weighs 4 kg, -1 m/s to 2 m/s - Change in KE = 1/2*4*(2*2-(-1)*(-1)) J = 6 J The answer says that the ranking is C, B, A, D, which doesn't make any sense to me given the calculations I made. Can someone please explain what I did wrong, or clarify the question? Quote Link to comment Share on other sites More sharing options...
FizziksGuy Posted November 28, 2016 Share Posted November 28, 2016 From the AP Physics 1 Essentials book... a tricky question. For B, because you're passing through the zero point, you find the energy going from 3 m/s to 0 m/s, and then the energy going from 0 m/s to -3 m/s, and add them. Same idea with D. It requires you to think past the standard definition and understand what sort of work must be done to accomplish the feat. Quote Link to comment Share on other sites More sharing options...
Lionblaze16 Posted November 29, 2016 Author Share Posted November 29, 2016 I see, it is the ENERGY REQUIRED TO CHANGE THE VELOCITY, not the OVERALL ENERGY CHANGE... Quote Link to comment Share on other sites More sharing options...
AngeFlight Posted April 15, 2020 Share Posted April 15, 2020 Oh, so you keep all the values positive. Thanks! Quote Link to comment Share on other sites More sharing options...
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