Exam 2
HW 5: What mass has a rest energy of 100J?
1.1x10⁻¹⁵kg
HW 6: Atmospheric pressure is about 100000 Pa. At what depth in water does this pressure double?
10 m
HW 5: The relativistic energy of a 1000 kg car moving at 0.95 times the speed of light is:
2.9x10²⁰J
HW 6: Water weighs about 10 N per liter on Earth. A 4.0 liter ball is pushed underwater on the space shuttle, where gravity was 1/10 that on Earth. That is the buoyant force on the ball?
4 N
HW 5: What is the gravitational force that two 100kg persons exert on each other when standing 1 m apart?
6.672x10⁻⁷N
HW 5: How far apart would two 100 kg persons need to be so that the force they exert on each other is equal to 1N? You can assume they are point masses, having mass but no size.
8.17x10⁻⁴m
HW 7: The diameter of a pipe is tripled while the pressure difference across the pipe remains the same. The volume flow rate of the pipe increases by a factor of
81
HW 5: The relativistic momentum of a 1000 kg car moving at a 0.95 times the speed of light is:
9.13x10¹¹kgm/s
PROBLEM: If you and a friend are separated by 10m and you both have a mass of 70kg, what is the magnitude of the gravitational force you exert on each other? F = G m₁m₂/r²
= (6.67*10⁻¹¹Nm²/kg²)(70kg)(70kg)/(100m²) F = 3.27*10⁻⁹N (rockets 17)
Why doesn't the atmosphere fall or collapse?
A gradient in its pressure supports its weight. (ballons 11)
Why are there different wing shapes?
Airspeed and performance influence wing design (airplanes 17)
What makes a spaceship orbit the earth?
An orbiting spaceship falls but misses the earth (rockets 18)
If there were no launch pad beneath a NASA rocket at liftoff, the upward thrust of its engines would be
Approximately unchanged
PROBLEM: The two feet of a 60kg person cover an area of 500 cm². a) Determine the pressure exerted by the two feet on the ground b) If the person stands on one foot, what will the pressure be under that foot?
Balloons 9 for solution
Why do balls experience air resistance?
Balls interact with and transfer momentum to air (balls and air 5)
HW 5: When moving, a bicycle is in a state of dynamic equilibrium due to which of the following?
Both gyroscopic effects and the shape of the fork
HW6 : Why will a rolled up ball of aluminum foil float in water but solid aluminum will sink?
Due to the air inside, the average density of the ball is lower than water.
Who was the creator of a special theory of relativity?
Einstein
When you approach a light source, the light from that source has a speed
Equal to c
As a planet moves in an elliptical orbit around its star, its speed is
Faster when it is closest to the star.
Pressure is
Force divided by area
HW 5: The maximum speed a rocket can achieve is
Greater than the speed of its exhaust plume but less than the speed of light
HW 7: When a fluid is in a steady state flow and its path bends, the pressure on the outside of the bend is always
Higher than the pressure on the inside of the bend.
HW 7: At what place in or near the jet engine is gas moving the fastest relative to the flying airplane?
In the plume of exhaust gas flowing out of the engine's outlet duct
Why does a moving tricycle flip during turns?
Inertial effects overwhelm its static stability (Bicycles 22)
When you drink water through a straw, the water
Is pushed upward by a pressure imbalance
Why does water move through level pipes?
It accelerates toward lower pressure (water distribution 5)
Why can you ride a bicycle without hands?
It automatically steers under center of gravity (Bicycle 36)
How does an airplane support itself in the air?
It deflects air downward; air pushes it upward. (airplanes 6)
How much does the diameter of a hose matter?
It matters a surprisingly large amount (Watering 9)
How does a plane propel itself through the air?
It pushes air backward with its props or engines (airplanes 24)
How does the rocket use gas to obtain thrust?
It redirects gas's thermal motion into a directed jet (rockets 10)
How does a plane turn?
It uses life to accelerate in the direction of turns (airplanes 21)
Why does a helium balloon float in the air
It weighs less than the air it displaces (ballons 25
Why does a hot air balloon float in cold air?
It weighs less than the air it displaces. (Balloons 21)
What pushes a rocket forward?
It's gaseous exhaust pushes it forward. p(fuel) + p(ship) = 0 (Rockets 7)
A hot air balloon rises because
Its average density is less than air
If water in a level pipe begins to move faster
Its pressure decreases
If water in a uniform pipe begins to flow uphill,
Its pressure decreases
Why does the atmosphere push up on a balloon?
Its pressure gradient pushes the balloon upward (Balloons 17)
How does air inflate a rubber balloon?
Its pressure pushes the balloon's skin outward. (balloons 4)
Suppose you have gotten a new job where you have to design balls that fly through the air with as little resistance as possible. You know that in front of the ball there is laminar flow, so in back of the ball there should be
Laminar flow, so the leading and trailing pressure environments are similar
HW 5: Compared to planets close to the sun, and based upon Kepler's third law of planetary motion, the length of the year of planets farther out from the sun are
Larger
HW 7: When a fish hovers over the bottom of a lake, what forces act on it, and what is the net force?
Lift and weight, for a net force zero.
Water pours gently from an open hose because it
Lost energy throughout its trip to the hose end
A smooth, gentle river flows past a cylindrical post. At the sides of the post, is the water level higher, lower, or equal to its level in the open river?
Lower
How would raising the height of a sport utility vehicle affect its turning stability?
Make it more likely to tip over.
Why do pipes rattle when you close the faucet?
Moving water carries momentum (watering 25)
As you ride in a jet airplane, the clouds are passing you at 600 mph. The air that has just entered part way into the intake duct of one of the huge jet engines is traveling
Much slower than 600 mph
The ideal gas law says
Pressure is proportional to absolute temperature
How can you produce pressurized water?
Push inward on the eater, using a surface (water distribution 6)
HW 5: Rockets receive their forward propulsion through
Pushing against the exhaust
Water flows through two tubes, one twice the diameter of the other. Both tubes have the same pressure differences between start and finish. The bigger tube will deliver water.
Sixteen times as quickly
PROBLEM: Liquid nitrogen has a boiling point of -195.81 °C at atmospheric pressure. Express this temperature in a) Degrees Fahrenheit and b) Kelvin
Solution: a) temperature in °F_ [(9/5) x (temperature in °C) ] + 32 temperature in °F = [(9/5)(-195.81) +32] °F = 320.5 °F. b) temperature in K = (-195.81 + 273.15) K = 77.34 K (balloons 24)
PROBLEM: Ip, a satellite of Jupiter, has an orbital period of 1.77 days and an orbital radius of 4.22 x 10⁵ km. From these data, determine the mass of Jupiter.
Solution: In this problem the massive planet Jupiter takes the sun (Rockets 26 for mathematical solution)
PROBLEM: A 2kg block of wood is floating in water. What is the magnitude of the buoyant force acting on the block?
Solution: The block is floating, it is not accelerating, the net force must be zero. So the magnitude of the buoyant force must be equal the weight. F(B) = mg = 19.6 N
How does air flow around a ball?
That depends on Reynolds number (Balls and air 7)
How does the airplane "lift off" the runway?
The airplane sheds a vortex and is lifted upward (airplanes 8)
HW 5: In studying many martial arts, it is necessary to practice efficient and safe postures. One important concept related to how one should stand is that the feet should be reasonably apart, because
The base of support is wider, thus stabilizing the person.
Why is stationary bicycle so unstable?
The bicycle is an unstoppable equilibrium. (Bicycles 16)
HW 7: Which of the following would cause a household fan to stall?
The blades are too sharply angled for the speed the fan rotates AND The fan is rotating too fast
As water flows, what happens to its energy?
The energy is converted between several forms (water distribution 16)
Why does water pour gently from an open hose?
The free-flowing water wastes most of its energy (watering 12)
You are riding a bicycle and lean to the left while taking a curve at a constant speed.
The net force on you is to the left
Why does water spray fast from a nozzle?
The nozzle causes water to turn PPE into KE (watering 13)
If you squeeze a closed bottle filled with water,
The pressure increases by the same amount of everywhere in the bottle
Considering the airflow around an airplane's wings,
The pressure is lower above the wing
As water flows through a bend in a hose, at the inside of the bend
The pressure is lower and the speed is higher
What limits a spaceship's speed, if anything?
The rocket's fuel to spaceship ratio (rockets 12)
HW 6: When driving down a level smooth road, your car tires flatten at the bottom and they nicely support the weight of your car. However when you run over a small pointed rock the tire is punctured and fails. The rock is able to puncture the tire because
The small surface area of the rock times the pressure in the tire is not enough force to support the car. Thus, the tire fails.
Why is a stationary tricycle so stable?
The tricycle is n a stable equilibrium. (Bicycles 12)
Why does plane tilt up to rise; down to descend?
The wing's angle of attack affects its lift. (airplanes 14)
HW 5: Astronauts in the space shuttle feel weightless because
They are in a constant state of free-fall.
Why do spinning balls curve in flight?
They experience two aerodynamic lift forces. (balls and air 22)
HW 5: Suppose you are spending your weekend on roller skates trying to get momentum from somewhere. Which of the following will give you momentum in the north direction?
Throwing your keys southward
Why must you lean a bicycle during turns?
To balance inertial effects with static instability (Bicycle 29)
Where does the work you do pumping the water go?
To the water at the delivery-end of the pipe (water distribution 13)
If a tricycle is on a sloping floor, will this rule about it being in a stable equilibrium as long as its center of gravity is above its base of support still work?
True
Why can't a commercial jetliner keep increasing its angle of attack to rise faster?
Turbulence will develop behind the wings
What causes hissing in a faucet, hose, or nozzle?
Water can become turbulent and produce noise (watering 19)
HW 6: Why are failures in dams more likely to occur closer to the bottom of the dam?
Water pressure is higher at the bottom.
How does a faucet control flow?
Water's energy and viscosity limit the flow. (Watering 4)
HW 6: In a popular classroom demonstration, a cotton ball is placed in the bottom of a strong test tube. A planner fits inside the tube and it makes an air - tight seal. It is then pushed down very rapidly, and the cotton flashes and burns. This happens because
When the air was compressed its temperature increased.
A helium balloon has mass, yet it doesn't fall to the floor. Is there a real force pushing up on the helium balloon?
Yes
Once in space, does a spaceship have weight?
Yes, but less than at ground level. (Rockets 15)
HW 7: A curve ball's path bends to the right as it flies toward home plate because it is experiencing an aerodynamic force to its right. One reason why the ball is experiencing this aerodynamic force is that the ball is spinning and that spin
causes the air to flow more rapidly around the right side of the ball than around the left side of the ball.
HW 7: Aerodynamic forces consist of
drag and lift
HW 7: As the Reynolds number decreases,
flow is going from being viscous dominated to inertia dominated and therefore turbulent.
Hw 6: Suppose you have two containers with liquid in them. One has a density of 760 kg/m³ and the other has a density of 910 kg/m³. If an object floats in one container and sinks in the other, the density of the objet
has a value between 760 kg/m³ and 910kg/m³
HW 7: Flowing honey is less likely to become turbulent than flowing water because
honey's large viscosity flavors laminar flow
An upright trickle is in a stable equilibrium because tipping it
increases its potential energy
A propeller works because
it generates lift forces in the forward direction.
HW 5: An object is in stable equilibrium only when
its total potential energy rises whenever it reorients.
The airflow around a slow-moving ball is ____ while the airflow around a fast-moving ball is ____ .
laminar; turbulent
HW 7: A curve ball in baseball curves because of
lift forces
HW 7: A fan can be used to circulate air around a room. The pressure at the inlet side of the fan is
more than the pressure at the outlet side.
HW 7: If a golf ball were smooth rather than dimpled, it would
not travel as far after being hit by a golf club
HW 6: Gravity creates
pressure gradients in fluids
HW 5: A three-wheeler is
statically stable and dynamically unstable
HW 5: A motorcycle is
statically unstable and dynamically stable
HW 7: Fluids have viscosity because
the atoms and molecules within the fluid interact with each other, producing internal frictional forces
HW 6: You are riding in a hot air ballon. You have not used the burner for some time and the balloon begins to sink. Assuming that no air is allowed to leave or enter the balloon,
the average density of the balloon became greater than the surrounding air and the buoyant force on the balloon became less than its weight.
When water flows through a pipe and the pipe becomes smaller in diameter,
the pressure decreases and the speed increases
HW 6: For an object that is floating on a fluid,
the weight of the fluid displaced equals the object's weight
HW 6: For an object that is neutrally buoyant in fluid,
the weight of the fluid displaced equals the object's weight.
HW 6: For an object that sinks in a fluid,
the weight of the fluid displaced less than its weight
A curveball works because
there are both a Magnus force and a wake deflection force acting on the ball.
The dimples on a gold ball help the ball fly farther because
they trip the boundary layer into becoming turbulent.
HW 7: The four things that affect the amount of water flowing through a hose are
water viscosity, hose length, inlet and outlet pressures and hose diameter
