PHY 1151

Summer 2003

**Third Hour Exam**

July 2, 2003

Statistics

High: 100 (2!!)

Mean: 84

Low: 40

BRAVO!I was very pleased with your results on this test. You did a good job!

I = (^{1}/_{2}) M r^{2} = (^{1}/_{2})
(4 kg) (0.1 m)^{2}

I = 0.02 kg m^{2}

A 2 kg mass is suspended from a string or cord that is wrapped around the solid
disk as shown here. The solid disk rotates as the cord unwinds so there is no
slipping between the disk and the cord.

**What is the acceleration of the hanging mass?**

2. A 10-cm cube of **wood** floats in water with 2 cm above
the water and 8 cm below the water (as shown in the sketch here). What is the
**density** of the wood?

(H_{2}O)
= 1000 kg/m^{3}

(H_{2}O)
= 1000 kg/m^{3}

Young’s modulus for steel is Y = 20 x 10

i.Think of a diver doing a double sommersault. He leaves the diving board hardly rotating at all. During most of his fall, he pulls his arms and legs in and spins very quickly. Before he enters the water, tho’, he extends his body and greatly slows his rotation to make an elegant and graceful entry into the water. Explain what happens in terms of thephysics principleswe have discussed recently.

Angular momentum is conserved. Angular momentum is the product of moment of inertia multiplied by angular velocity. Initially outstretched, the diver has a large moment of inertia. As he pulls himself into a “tuck” position, his moment of inertia become small and that requires that his angular velocity becomes large. Before entering the water, he extends his body and that makes his moment of inertia large. A large moment of inertia requires a small angular velcoity.

ii.A solid disk and an open hoop are released from the same height on an inclined plane. Which reaches the bottom first? And?why

The solid disk has a smaller moment of inertia. That means it is easier to rotate so it comes down the inclined plane faster — and first!

iii.Why do people commonly refer to theof a ship as its “displacement”?weight

The bouyant force provided by the water is equal to the weight of the waterdisplacedby the ship.

iv.As air moves across the top of a chimney, will air be pushed down into the chimney and into the house or will air be pulled from the house and up and out of the chimney.Why?

From Bernoulli’s equation, we know that the pressure of a moving fluid islessthan the pressure of the fluid at rest. Thus the air pressure at the top of the chimney, where the air is moving, islessthat the air pressure inside the house, at the fireplace. Therefore, air will move from inside the house, through the fireplace, up the chimney, and out into the moving air.