Download 8th Grade - Physics

8th Grade Core Science Vocabulary: Physics
Archimedes’ Principle states that the buoyant force acting on a submerged object is
equal to the weight of the volume of fluid that has been displaced by the object. Partly
submerged objects displace a volume of fluid equal to the portion of the object that is
submerged.
If an object displaces 50 liters of water in a swimming pool. The buoyant force on the
object will be equal to the weight of 50 liters of water, or about 500 newtons. (pg. 428)
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Archimedes'
Principle
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buoyant
force
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density
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energy
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force
The upward force exerted by water and other fluids on a submerged object. The
buoyant force acts in the direction opposite to the force of gravity, so it makes the
object feel lighter. The less dense the object is, the greater the buoyant force it
experiences. Since the pressure in a fluid increases with depth, the upward pressure on
the bottom of the object is greater than the downward pressure on the top. The result
is a net force acting upward on the submerged object, this is the buoyant force.
(pg. 427)
The ratio of the mass of a substance to its volume, or mass per unit volume. The
formula for finding density is:
density = mass
d = m
volume
v
A 15 cubic centimeter sample of aluminum has a mass of 40.5 grams.
Therefore, the density of aluminum is 2.7 g/cm3 (pg. 425)
Work is done when an object is caused to move a certain distance. The ability to do the
work or cause change is called energy. Work is the transfer of energy. The SI unit for
both work and energy is the joule (j). Examples of energy include: kinetic, potential and
mechanical. (pg. 358)
A push or a pull exerted on an object. A force is described by its magnitude and by the
direction in which it acts (vector). You can use an arrow to represent the direction and
strength of a force. The arrow points in the direction of a force and the length of the
arrow indicates the strength of a force. The SI unit for the magnitude or strength of a
force is the newton (N). When an object is subject to two or more forces at once, the
result is the combination of all the forces. This is called the net force. (pg. 375)
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friction
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gravity
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inertia
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momentum
The force that one surface exerts on another when the two surfaces rub
against each other. Friction acts in a direction opposite to the motion of
objects. Smooth surfaces produce less friction than rough surfaces.
There are 4 types of friction: (pg. 381)
Static friction is the friction that acts on objects that are not
moving.
Sliding friction occurs when two solid surfaces slide over each
other.
Rolling friction occurs when an object rolls across a surface.
Fluid friction occurs when a solid object moves through a fluid.
The force that acts to pull objects straight down toward the center of the earth. Gravity
is the force that pulls objects toward each other. The force of gravity between objects
increases with greater mass and decreases with greater distance. (pg. 384, 474)
The tendency of an object to resist any change in its motion. Newton’s First Law of
Motion is called the law of inertia. Example: When a car stops, inertia keeps the
passengers moving forward. A seat belt is required to change the motion and to keep
the passengers in their seat. Inertia is dependent on mass. The greater the mass of an
object, the greater its inertia and the greater the force required to change its motion.
(pg. 390, 476)
The ‘quantity of motion’ or the product of an object’s mass and velocity. The
momentum of a moving object can be calculated by multiplying the object’s mass and
velocity. The unit of measurement for momentum is kg∙ m/s. (pg. 396)
Momentum = Mass x Velocity p = mv (the abbreviation for momentum is “p” Nobody
knows why!)
Example: Which has more momentum: a 3.0 kg sledgehammer swung at 1.5 m/s, or a
4.0 kg sledgehammer swung at 0.9 m/s?
Smaller sledgehammer- p = 3.0 kg x 1.5 m/s = 4.5 kg∙ m/s
Larger sledgehammer- p = 4.0 kg x 0.9 m/s = 3.6 kg∙ m/s
The smaller sledgehammer has more momentum because it has greater velocity.
The force exerted on a surface divided by the total area over which the force is exerted.
The larger the area over which the force is distributed, the less pressure is exerted for
each unit of area. Example: You can lie on a bed of nails without harm because you are
distributing the force over a large area, but if you lay on a single nail, all of the force is
distributed to one small area. (pg. 417)
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pressure
Pressure = Force / Area
The SI unit of pressure is the newton per square meter (N/m2). This unit of pressure is
called the Pascal (Pa) 1 N/m2 = 1 Pa
The distance an object travels per unit of time. To calculate the speed of an object,
divide the distance the object travels by the amount of time it takes to travel that
distance. The formula for calculating speed is:
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speed
speed = distance
time
s=d
t
Example: a cyclist who travels 30 km in 2 hrs has a speed of 15 km/hr. (pg. 342)
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vector
velocity
Force in a given direction. A measurable quantity that consists of both magnitude and a
direction. Other examples of vectors include velocity, acceleration and force. Vectors
can be shown graphically by an arrow that represents the direction and strength of the
force. The arrow points in the direction of a force and the length of the arrow tells you
the strength of a force. (pg. 341)
Speed in a given direction. It has both magnitude (speed) and a direction. Example:
the velocity of a storm can be described as moving 25 km/hr to the east.
The same formula for speed is also used to calculate velocity: Velocity = Distance/ Time.
(pg. 344)