1. Your friend has a pie on the roof of his van. You are standing on the ground

Question

1. Your friend has a pie on the roof of his van. You are standing on the ground and observe the van stopping abruptly for a red light. The pie does not slip off the roof. (a) Construct a motion diagram and a force diagram for the pie as the van approaches the red light, from your frame of reference and from the driver's frame of reference. (b) Repeat part (a) for the case when the light turns green. Be sure to specify the observe and identify the object causing each force. (c) Are the motion diagrams consistent with the force diagram for each case?

2. Spider- Man holds the bottom of an elevator with one hand. With his other hand, he hold a spider cord attached to a 50-kg of explosives at the bottom of the cord. Determine the force that the cord exerts on the box if (a) the elevator is at rest; (b) the elevator accelerates up at 2.0 m/s2; (c) the upward - moving elevator's speed decreases at a rate of 2.0 m/s2; and (d) the elevator falls freely.

3. Astronaut Karen Nyberg, a 60-kg astronaut, sits on a bathroom scale in a rocket that is taking off vertically with an acceleration of 3 g. What does the scale read?

4. You push a bowling ball down the lane toward the pins. Draw force diagrams for the ball (a) just before you let it go; (b) when the ball is rolling (for two clock readings); (c) as the ball is hitting a bowling pin. (d) For each force exerted on the ball in parts (a) - (c), draw the Newton's third law force beside the force diagram, and indicate the object on which these third law force are exerted.

5. EST Olympic dive During a practice dive, Olympic diver Fu Mingxia reached a maximum height of 5.0 m above the water. She came to rest 0.40 s after hitting the water. Estimate the average force that the water exerted on her while stopping her

Explanation / Answer

2. mass =50kg

a)  the elevator is at rest;

force =mg =50*9.8=490N

b) the elevator accelerates up at 2.0 m/s2 :

force =mg +ma =490+50*2 =590 N

c) the upward - moving elevator's speed decreases at a rate of 2.0 m/s2;

force =mg-ma =490 -50*2 =390N

d) the elevator falls freely.

force =mg-mg =0 N

3. mass =60 Kg

scale read =m(g+a)=60(g +3g) =60(4*9.8) =2352 N , 240 kg

5. Working formula is

M(Vf - Vo) = F(delta T) --- call this Equation 1

i.e., change in momentum of the diver is equal to the impulse.

where

M = mass of the diver
Vf = velocity of the diver as he enters the water
Vo = initial velocity of diver = 0
F = force exerted by the water on the diver
delta T = 0.4sec (stopping time of diver in the water)

To solve for "Vf", use the law of conservation of energy, i.e.,

Potential energy = Kinetic energy

Mgh = (1/2)(M)Vf^2

where

h = initial height of diver above water = 5 m (given)
g = acceleration due to gravity = 9.8 m/sec^2 (constant)

and all the other terms have been previously defined.

Since "M" appears on both sides of the equation, then it will simply cancel out, hence the above becomes

gh = (1/2)Vf^2

Vf^2 = 2gh

and substituting values

Vf^2 = 2(9.8)(5)

Vf = 9.9 m/sec.

Substituting values into Equation 1,

M(9.9 - 0) = F(0.4)

Solving for "F",

F = M(9.9)/0.4

F = (24.5M) N

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