Definitions
Units & Measurement
Density Logic
Pressure & Proportions
Kinetic Theory of Matter
100
What is the equation used to calculate density?
Density = mass / volume
100
The SI unit commonly used to measure mass in our lab.
grams
100
Why does hot air rise?
because it is less dense than cool air
100
What is the equation used to calculate pressure?
Pressure = Force / Area
100
A pot of water is placed on a stove and heated up until it starts to boil. What happens to the motion of the water molecules inside the pot?
The molecules start moving faster and faster as they heat up.
200
A push or a pull that can change an object’s motion or shape.
Force
200
Dr. Bartley calculates the density of an object and gets 2.7 as the value. Why would reporting density as just “2.7” be incomplete?
Always include units!!!
200
If two objects have the same volume, the one with greater mass has ____ density.
greater
200
If force stays the same and area increases, pressure will _____.
decrease
200
Dr. Bartley gets frustrated at the beach ball we used to represent the sun in our solar system lab. So she tosses it outside into a big pile of snow and leaves it there overnight. What would happen to its volume and why?
Its volume would decrease because the gas molecules inside the beach ball would slow down as they get colder, thus exerting a smaller force on the inside wall of the ball. This causes the volume to shrink, even though the number of particles inside the ball stays the same.
300
What is the difference between mass and weight?
Mass is the amount of matter in an object. Weight is how strongly gravity pulls on that matter.
300
A common unit used to measure pressure in class demonstrations.
an atmosphere (atm)
300
Oops! You make a mistake and measure an object's volume as larger than it actually is. Assuming you correctly measured the mass, the density you calculate will be _____ than it actually is.
smaller
300
Inside a vacuum chamber, once air has been expelled, the pressure is ______ than the air pressure outside the chamber. Explain why.
The pressure is lower inside the vacuum chamber because air has been expelled, which means the force from the air on the inner walls of the chamber has significantly decreased, but the areas of the inner walls has stayed the same. This means the force decreases.
300
A "pressure cooker" is a sealed pot that uses high-pressure steam to cook food 3–10 times faster than normal. Modern pressure cookers have safety features built in to avoid accidents. Older or damaged models sometimes run into... problems. Explain why this accident happened using physics.
A pressure cooker works by increasing the temperature inside a sealed container. This would raise the gas pressure in a fixed volume, and if that pressure exceeds the cooker’s design limits, it fails catastrophically.
400
What is the difference between temperature and heat?
Temperature is an average measure that describes how fast the particles are moving in a substance. Heat is the amount of thermal energy a substance contains.
400
You run three trials where you measure the mass and volume of water to calculate its density. You get 0.95 g/mL, 1.03 g/mL, and .92 g/mL. What is the true value for the density of water and why did were you not able to calculate it exactly?
The density of water is 1 g/mL.
You cannot calculate it exactly because of measurement error! There is an inherent uncertainty in every measure you make - you can never measure a number exactly.
400
An object sinks in fresh water but floats in salt water. What must be true about its density?
It is between the densities of fresh and salt water.
400
If force decreases and area increases, pressure will _____. Why?
If force decrease and area increases, pressure will decrease.
P = F / A. This means that F is directly proportional to P and A is inversely proportional to P. If F decreases, P decrease. Similarly, if A increases, P decrease. Both changes lead to the same outcome: P decreases.
400
In class, we used a vacuum chamber to make a marshmallow get bigger. Why did the marshmallow expand?
The marshmallow expanded because the air pressure outside it decreased, so the air inside the marshmallow pushed outward more than the air outside, and the soft marshmallow material stretched and took up more space.
500
What is the kinetic theory of matter?
All matter is made of tiny particles that are always moving in random directions.
(The speed of their motion depends on temperature, and their collisions help explain pressure and volume.)
500
What are the three main reasons behind measurement error?
human error, inaccurate tools, environmental factors
500
Why is density is called a characteristic property of a material?
It does not depend on the shape, size, or amount of the material.
500
If force doubles and area doubles, pressure will _____.
Stay the same.
If both force and area double, then they change exactly in proportion, meaning P is unchanged, so the fraction P = F/A remains unchanged. Written out: 2F/2A = F/A = P
500
If you heat up water in a soda can and turn it upside down into a cool water bath, it implodes. Why does this happen?
When we heated the water in the soda can, the air and water vapor inside warmed up and moved faster. Some of the hot air escaped through the opening. When we turned the can upside down into cool water, the remaining air cooled quickly, and there were fewer molecules inside. This caused the pressure inside the can to drop. Because the outside air pressure was much higher than the pressure inside, it pushed inward and caused the can to implode.