Science 9
Name:_____________
Review Questions: Fluid Forces
Concept of Force
1) What is mass? What is matter? How can you relate these terms to density?
Anything that is composed of matter will occupy space, and mass is a measure of the quantity of matter.
For example, to say that something has a mass of 24 kg means there is 24 kg of matter present. The more
matter per unit of space, the higher the density.
2) According to your notes, how do things that have mass experience motion? Please give an example in your
response.
Anything that has mass will require a FORCE applied to it to change its motion.
Example: you have to push a desk for it to experience motion (you applied a force - push - to the desk)
3) What is weight? What type of force is associated with weight?
Weight is the amount of attraction between any objects that have mass - this FORCE of attraction is called
gravitational force. The larger an object, the greater the gravitational force that it will exert on another
object.
4) What is the difference between mass and weight?.
Mass refers to the amount of matter in an object, weight refers to the amount of gravitational force it
experiences.
5) Please list 5 types of forces, and give an example of each (different examples than we came up with in
class!)
Magnetic - e.g. a magnet
Friction - e.g. air resistance
Gravitational - e.g. waterfall
Elastic - e.g. slingshot
Electrical - e.g. static cling
Unbalanced Forces
6) What can we say about the forces acting on an object that is in constant motion?
If an object is in CONSTANT motion, the forces are BALANCED.
7) How can an object be moving but have no NET forces acting on it?
If an object is not experiencing acceleration, it is in constant motion, and the forces are BALANCED. When
forces are BALANCED there is no NET force.
Buoyancy
8) Please state Archimedes' Principle, as it applies to buoyant force.
Archimedes said the buoyant force on an immersed object is equal to the weight of the fluid displaced.
9) Why will an object weigh less when submerged in a liquid? Please draw a diagram and fully explain.
See notes for diagram. When an object is submerged in a liquid, there is a buoyant force that acts UP on
the object, causing it to weigh less (recall that weight is a measure of gravitational force - since the buoyant
force is acting UP, it is 'cancelling' out some of the gravitational force)
10) When you hold a rock submerged in water it will appear lighter, but why won't the same rock sitting at the
bottom of a lake appear just as light (before you 'pick it up')?
When the rock is at the bottom of the lake, there is no fluid UNDER the rock, and therefore no buoyant force
acting UP on the rock. Therefore it would essentially have the same weight (gravitational force) on the
bottom of the lake as it would on land.
11) Is it easier to plug a hole in a container from the outside of the container, or from the inside of the container?
Why?.
When water is pouring out of a hole, the PRESSURE of the water above it is forcing water out of the hole.
When you try to plug it from the outside, it's much more difficult as you are trying to work 'against' all that
pressure. When you plug it from the inside, the pressure of the fluid will work to plug the hole for you!
12) How does an understanding of buoyancy help explain how the density of objects determines whether they
will sink or float? Give a full explanation for why an object would sink, remain at the same level
underwater, or float above water.
If the weight of an object is larger than the buoyant force, and object will sink
If the weight of an object is equal to the buoyant force, an object remains at that level
If the weight of an object is less than the buoyant force, the object will float.
As objects become more dense, their weight will increase, therefore the denser an object is the greater the
tendency it will have to sink (and vice versa).
Pressure
13) If you apply the same force, will the 'sharp' end of a pencil or the 'blunt' end of a pencil be easier to jab
through a cardboard box? Explain why, and make sure you refer to pressure and area in your response.
If you apply the sharp end of a pencil with the same force, there will be more PRESSURE on the surface of
the box, than if you applied the same force with the blunt end. If the same force is applied over a greater
area, there is less pressure applied. For example, your feet exert more pressure on snow when you wear
boots than when you wear snow shoes (that's what keeps you from sinking).
14) Why is it accurate to say we are 'under a tremendous amount of pressure' on the surface of the Earth? What
force is causing this pressure? What is this force acting upon?
Recall that air is composed of molecules. The force of gravity acts on all of the molecules in our
atmosphere. Since the force of gravity becomes stronger as you near the center of the Earth, the force of
gravity is stronger on the surface of the Earth than in the clouds. When you are standing on the Earth, the
gravitational force of the Earth is 'pulling' on all those particles that are above you, and causing pressure
on your body! We are so used to it that we don't even notice.
15) Why does water boil at a lower temperature at higher altitudes?
There is less air pressure at higher altitudes. For water to boil, the liquid particles need to gain enough
energy to 'break apart' from each other. When these molecules are under less pressure, they are more free
to break apart, and therefore more free to boil!
16) Why is it fair to say that a vacuum cleaner doesn't 'suck' things up, and/or that there's no such thing as
'suction'?
Vacuum cleaners operate by creating a 'negative pressure' inside the vacuum bag. Air is forced out of the
vacuum which causes a change in pressure inside the vacuum compared with its surroundings (the vacuum
has a lower pressure, causing air particles from the higher pressure surroundings to enter the vacuum).
The difference in pressure forces air to enter the vacuum, causing the 'sucking'.
17) How does a change in volume affect pressure. Please give a clear example of a situation where the volume
of a container is decreased, and where the volume of a container is increased, and describe how this affects
pressure.
When you decrease the volume of a container, you increase pressure since you have forced the same
number of particles to 'exist' in a smaller space.
When you increase the volume of a container, you decrease pressure since you have allowed the same
number of particles to 'exist' in a larger space.
18) How does a change in temperature affect pressure. Please give a clear example of a situation where the
temperature of a container is decreased, and where the temperature of a container is increased, and describe
how this affects pressure.
Recall that temperature is the average velocity (speed) of particles in a substance.
Let us assume that the container is a CLOSED SYSTEM - no particles enter or leave.
When you increase the temperature, you increase the pressure since you have caused the particles to move
about at an increasing speed inside the same container.
When you decrease the temperature, you decrease the pressure since you have allowed the particles to slow
down inside the same container.
Basically any situation where you have less collisions will cause a decrease in pressure (and vice versa)
19) How does depth affect pressure. Please give an example of how a change in depth affects pressure.
When you go deeper in the ocean, you are getting closer to the center of the Earth, and the force of gravity
is getting stronger. More importantly, when you are deep in the ocean, you have A LOT of particles (of
water - and air) above you that are experiencing the Earth's gravitational force, and putting A LOT of
pressure on (top) of you!
20) Why will water flow out of two containers that are drastically different sizes, but filled to the same level
with water? (Check your notes for a diagram).
Check out your notes - it is the DEPTH of water that determines the amount of pressure, not the size of the
container.
Pascal's Principle
21) Using Pascal's Principle, explain why squeezing the 2L pop bottle caused the dropper to sink to the bottom
(this was a demo done in class)
When you squeeze the container, you increase the pressure inside the WHOLE container. This causes an
increase in pressure on the dropper. The increase in pressure causes the rubber part of the stopper to
'collapse' a bit, which decreases its volume. This means the dropper has experienced a decrease in volume
without a change in mass - it has become more dense! Since it is more dense, it now will sink to the bottom
of the 2 L bottle.
22) Please explain what is meant by 'mechanical advantage'.
Mechanical advantage refers to the ability of a device to multiply an effort force that someone puts into a
system - in other words, mechanical advantage allows you to apply a greater force than you yourself are
investing.
23) Please draw a diagram and explain how a floor jack works to move extremely heavy objects using relatively
little force.
Check your notes for this one - there's a pretty good explanation there too.