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Wonders of Physics

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Double Pivots Question

A man is standing on a uniform plank of mass 20 kg and 6 m long. He is standing 1 m from the string at A.


a) Find the unknown forces FA and FB.

The man now walks to the left of A.


b) Determine the maximum distance in which the man can move before the plank topples.


a) Fa = 550 N and FB = 250 N

b) d = 0.67 m

Refer to the video tutorial for the explanation.

Click on this post for the comics on moments concepts





N2010 P2Q12 Fuel gauge using variable resistor



View the video on how the gauge works.

<p><a href=”https://vimeo.com/138991946″>fuel guage using ammeter and variable resistor</a> from <a href=”https://vimeo.com/user10931667″>evantoh</a&gt; on <a href=”https://vimeo.com”>Vimeo</a&gt;.</p>

As the fuel level drops, the float which stays on the fuel surface will descend. The rod which is attached to the float will turn clockwise about the pivot X. As the rod turns, the resistance on the variable resistor increases. This increases the resistance of the circuit. Hence the current flowing through the circuit will decreases, causing the needle to deflect more to the left, indicating towards E (empty). Thus the reading on the fuel gauge decreases.



(i) Both the fixed resistor and sensor are in series.

Total effective resistance Re = 5000 + 1000 = 6000 ohms

V = IR
12 = I x 6000
I = 0.0020 A

Hence potential across Y, V = IR (where I is constant in a series circuit)
= 0.0020 x 1000
= 2.0 V

(ii) When the temperature increases and the resistance of sensor Y decreases, the total effective resistance of the circuit decreases. Current flowing through the circuit increases.

Since V = IR, where the R of the fixed resistor is a constant 5000 ohms, as current I increases, the potential difference across the 5000 ohms resistor will increase.