Mistake Master
Mistake Master · AP Physics C · Unit 3 · Topic 3.4 · Step-Through Animation
The Flat Line Energy Can’t Cross
You'll learnto read speed and turning points off a total-energy line across a potential curve.
Draw the total energy as one flat line across the potential-energy curve. Kinetic energy is just the gap up to it — the cart is fastest where the well is deepest, and turns around exactly where the curve rises to meet the line. Then friction quietly lowers the line.
ENERGY DIAGRAM
Before you start
What you're looking at
A 2 kg cart released from rest at the left rim of a smooth valley, with U = 100 J at the rim.
The question
Where along the track is the cart fastest, and where does it stop and turn around?
Watch for
The gap from the curve up to the flat energy line — that gap is the kinetic energy.
What the symbols mean
The curve is U(x); the flat line is total energy E; the gap K = E − U is the speed.
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