Units 5-6 Flashcards
What does a bathroom scale actually measure since it doesn’t measure your weight?
It measures the contact force between you and the floor (not your weight).
Stand on a scale in an elevator to see how it changes.
True or False: When a basketball player jumps horizontally to dunk the basketball, they stay in the air the same amount of time as if they had jumped straight up?
True
Which means that if you go really high vertically jumping, you can go further horizontally jumping
Explain how air friction works.
Air is just made up of a bunch of molecules with mass. You can figure out the mass (hypothetically) of each molecule and use that to calculate force and acceleration. Then when you consider how many air molecules their ARE, that calculated acceleration adds up quick and restricts whatever is pushing against it.
(Fly and train)
What is the one accelerating object on earth that is large enough to affect earth’s rotations?
The gravitational pull of daily ocean tides (caused by the moon).
Earth’s rotation slows because of friction with the water.
Fun fact: 400 million years ago, an earth day was only 22 hours long!
WOAH!
What is happening when something has reached terminal velocity?
Gravity balances air friction, so the net force and acceleration go to zero. Velocity remains constant.
Freefall
The act of always falling under the pure influence of gravity and NO OTHER FORCES
Are astronauts truly “weightless” in space?
No, because weight is affected by gravity, and they are still being pulled by earth (although at a slightly different amount because they’re hundreds of km farther away from earth’s center).
They’re just in a state of freefall while travelling with a rocket ship (at the same speed as the rocket). They have no other forces acting upon them.
What’s another name for a sideways force?
Centripetal
What is the centripetal force equation?
Force = (mass x speed^2) / radius
True or False: When being pulled inward by a centripetal force, a centrifugal force is pulling you outward
FALSE
The only force needed to create a circular motion is centripetal.
True or False: The stronger the sideways force, the tighter the turn
True
True or False: Friction is a centripetal force
True.
Different kinds of forces can act as a centripetal force.
Is force related to velocity?
NO
Force is wondering how an object’s motion is changing, not how fast or what direction the object happens to be going right now.
True or False: Air is classified as a gas
FALSE
It’s classified as a fluid because it flows
True or False: All gases are classified as fluids
True
Other than friction what are some of aspects of force that fluids create?
Gravity and pressure
What is the Pressure equation?
Pressure = Force / Area
What is pressure?
The total force on an object divided by the area over which the force is applied.
What is a fluid?
Anything that flows
Why don’t your feet hurt standing on them, but your toes hurt when you stand on tiptoes?
Because of pressure.
You’re exerting the same amount of force down on the ground (which is exerting that force up on you), but the pressure is dispersed over a smaller surface area (just your toes). That means your toes have to deal with all that contact force on them instead of having it spread out over the whole feet.
Why are high heels bad for the floor?
Because you’re exerting a high force over a teeny tiny area, which greatly multiples the total pressure. Materials being walked on are often not strong enough to support that much pressure.
That’s also why high heels break. The floor is exerting the same force up onto a tiny area, and the point of the heel must be strong enough to endure that pressure.
What are “bounded” fluids?
They are wholly contained. Like a water balloon or water bottle.
What are “unbounded” fluids?
Fluids that lack boundaries on one or more sides, like the oceans or the atmosphere.
True or False: Unbounded fluids are pulled toward the earth and held in place by gravity.
True
What would an unbounded fluid do if allowed to continue flowing?
They would move downward until there was nowhere else for them to go. Then the flow would stop.
What are the four rules of unbounded fluids?
- Pressure depends on depth only and is greater at greater depth.
- Pressure is the same for all points at the same depth
- Pressure at a given depth is independent of direction
- Pressure is always perpendicular to the surface of a submerged object
True or False: Pressure depends on volume of a fluid
FALSE
True or False: Pressure depends on surface area of a fluid
False
How is weight measured vertically in a fluid?
You look to the bottom most measure, and just take the weight of that and everything on top of it.
The column. You weigh the entire column, so the depth you are at, and everything above it equals the weight of that spot.
What would happen if pressure changed from place to place at the same depth?
The fluid would flow from high pressure zones to low pressure zones, reducing the differences to zero and stopping the flow.
What is the buoyant force?
A force pushing upward on objects immersed in a fluid.
True or False: The force of gravity changes on an object when it is submerged in water
False
What is the Archimedes’ Principle?
An object immersed in a fluid experiences an upward buoyant force caused by contact interactions with the surrounding fluid.
The strength of this force equals the weight of the displaced fluid.
What causes the buoyant force?
Contact interactions with the surrounding fluid.
What is the strength of the buoyant force?
It is equal to the weight of the displaced fluid.
What is the buoyant force on a ball of water in surrounding water?
There is a net zero force because the buoyant force exactly equals the weight of the water ball.
What are the two forces that impact an object in a fluid?
Buoyant force pushing up and gravity pulling downward.
When will an object sink?
When the force of gravity exceeds the buoyant force.
What rule exists to determine whether solid objects float or sink?
DENSITY
What is the density formula?
Density = mass / volume
True or False: If a solid object has a greater density than the fluid in which it is placed, it will sink.
True
True or False: If a solid object has a smaller density than the fluid in which it is placed, it will float.
True
True or False: Density of an object matters more than its size when placed in a fluid.
True
What is the relationship between density and the buoyant force?
The greater the difference between the density of the object and the density of the fluid, the higher the object will float.
When would an object float or sink depending on its shape?
When the object is not solid all the way through.
Why do hot air balloons rise?
Because they overcome the pull of gravity using air inside of them that is less dense than the air outside.
Why is the air inside a hot air balloon less dense than the air around it?
Because it is heated air, it is expanded
True or False: Water, like most liquids, has essentially the same density wherever it is, regardless of being in the depths of the ocean or in a lake.
True
True or False: Buoyant forces change with depth
FALSE
True or False: Air decreases in density with height.
True
Why will an object continue sinking to all depths but an object rising in the atmosphere will eventually hover and stop?
Because buoyant forces are the same no matter what depth the object is in. But the density of air decreases with height, so the forces eventually balance out.
True or False: Continents float deeper in the mantle than oceans
True
Why do continents sink lower in the mantle than oceans, even though they are lighter?
Because continents are thicker and have greater total weight than oceans.
True or False: The taller a mountain or the more massive the continent, the deeper the roots.
True
What is convection?
The process by which energy is moved from one place to another by being stored in matter as internal energy, then moving the matte from one place to another.
I.E. Circulation in a fluid caused by differences in temperature and density
Hot/cold air densities that cause movement.
Hot air rises, cold air sinks, etc.
What is a really good example of how convection works?
Lava lamps. The waxy material floats up away from the heat source, cools, then sinks, then heats again and rises, etc.
Summarize the Archimedes principle:
An object immersed in a fluid experiences and upward buoyant force caused by contact interactions with the surrounding fluid. The strength of this force is equal to the weight of the displaced fluid.
What is density?
How much mass there is in a certain amount of volume.
Why don’t scientists talk about a “piece” of air or liquid?
Because fluids don’t have set boundary lines, atoms move around a lot. They don’t have rigid shapes.
True or False: Pressure depends on the characteristics of the object placed in the fluid
False
True or False: Pressure depends on the characteristics of the fluid that the object is placed in
True
True or False: Pressure is always perpendicular to the surface of a submerged object (or to the walls of the container).
True
How much force is on us from the atmosphere (air pressure)?
14.7 lbs/square inch
Why don’t we feel the effects of air pressure?
Because the forces in our bodies balance it out.
What is neutral buoyancy?
When the buoyant force and gravity cancel each other out so the object remains suspended in the fluid.
True or False: Something can be neutrally buoyant in the middle of the fluid (like below water).
True
What is a BCD?
A buoyancy compensation device that scuba divers wear
What are four different ways to say an object is floating?
- It is less dense than the surrounding fluid
- It weighs less than an equal volume of fluid weighs
- The buoyant force is equal to its weight and it is only partially submerged
- It displaces a weight of fluid equal to its weight when it is partially submerged.
True or False: Buoyant force must be compared to weight.
True
True or False: The buoyant force is always upward
True
What is boiling water an example of?
Convection
