# Essay on Homework #2 Solutions

HOMEWORK #2

Due Friday June 22, 2012

1

A 70.0-kg person stands on a scale placed on the ﬂoor of an elevator. Find: - the weight of the person (magnitude and direction), - the normal force by the scale on the person (magnitude and direction), - and what the scale reads (in kilograms) in the following cases: (a) The elevator moves up with a constant speed of 2.0 m/s2 . (b) The elevator has a constant upward acceleration of 2.0 m/s2 . (c) The elevator has a constant downward acceleration of 2.0 m/s2 . (d) The cable snaps and the elevator falls freely (ignore friction and the bloody end!). SOLUTION: The weight of the person depends on Mass and g, therefore: W = M g = (70.0 kg) 9.80 m/s2 = 686 N downward in all

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θ = 25◦

F = 12 N x

25◦

12 N Mg SOLUTION : Since the system is in equilibrium, then we set a to zero in newton’s second equation: x − direction : ⇒ y − direction : ⇒ Solve equations 4 and 5 for M : M= = F g tan θ 12 N 9.80 m/s2 tan (25◦ ) Fx = 0 F = T sin θ Fy = 0 M g = T cos θ (5) (4)

F − T sin θ = 0

T cos θ − M g = 0

= 2.6 kg

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Physics 221 Summer 2012

HOMEWORK #2

Due Friday June 22, 2012

8

In a famous scene of a movie, Indiana Jones is dragged behind a Nazi truck by his whip (the rope in the ﬁgure). Jones’ mass is 90 kg and his whip makes an angle of 10◦ with the horizontal. The coeﬃcient of kinetic friction between his body and the ground is 0.50. Find the tension on the whip when the truck is driving at a