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The moment of the ladder about the floor support point is zero. The 98 N ladder weight acts downward 0.4 m from the wall.
The wall contact point is sqrt[2.5^2-0.8^2] = 2.37 m above the floor. The force F at the wall contact point is normal to the wall, because it is frictionless there.
Setting the total moment equal to zero,
98*0.4 = F*2.37
F = 16.54 N
A 10-kg ladder 2.5m long rests against a frictionless wall with its base on the floor 80cm from the wall. How much force does the top of the ladder exert on the wall?
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