November 26, 20169 yr 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?
November 28, 20169 yr 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.
November 29, 20169 yr Author I see, it is the ENERGY REQUIRED TO CHANGE THE VELOCITY, not the OVERALL ENERGY CHANGE...
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