Concept #1: Forces with Vertical Acceleration

Practice: The system shown below is pulled up with a constant 100 N. Calculate the tension between the blocks. Calculate the acceleration of the system (using +/- to indicate if it is up or down).

Practice: You are inside a bucket that is connected to a pulley above you by a vertical rope. You pull yourself up by pulling down on the other end of the rope. If the total mass of you plus bucket is 80 kg, how hard must you pull down on the rope to move up with a constant 2 m/s^{2} ?

Practice: A 70 kg diver steps off a board 9 m above the water and falls vertical to the water, from rest. If his downward motion is stopped 2.0 s after his body fist touches the water, what average upward force did the water exert on him?

A skydiver feels "weightless" during free fall because
A) there is no normal force acting on the skydiver during the free fall.
B) there is no gravitational force acting on the skydiver during free fall.
C) there is no net force acting on the skydiver during free fall.
D) the gravitational force acting on the skydiver is canceled by air resistance.

In an elevator with a downward acceleration, the normal force acting on a box of mass m resting on the elevator floor is
A) equal to mg.
B) less than mg.
C) greater than mg.
D) zero.

An elevator begins its ascent by accelerating at 2 m/s2 until it reaches its appropriate ascent speed. If the elevator has a mass of 1000 kg, and the cables supporting the elevator can withstand a maximum tension of 41.3 kN without snapping, what is the maximum mass of people that can safely ride the elevator?
A) 3,200 kg
B) 4,200 kg
C) 2,000 kg
D) 2,500 kg

What should be the acceleration of an elevator that makes you feel weightless? Assume that the positive y-axis points up.

Consider a man standing on a scale which is placed in an elevator. When the elevator is stationary, the scale reading is his weight W. Find S, the scale reading when the elevator is moving downward with acceleration a = 1/6 g.
1. S = 8/7 W
2. S = 6/5 W
3. S = 5/7 W
4. S = 0 m/s2
5. S = 7/6 W
6. S = 4/3 W
7. S = 7/5 W
8. S = 5/6 W
9. S = 6/7 W
10. S = 2/3 W

A 55-kg person steps on a scale in an elevator. The scale reads 460 N. What is the elevator doing?[A] The elevator is in free fall.[B] None of these.[C] It is accelerating downward at 1.44 m/s2.[D] It is stationary.[E] It is accelerating upward at 0.41 m/s2.