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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) 1 Archimedes' Principle 2 buoyant force 3 density 4 energy 5 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) 6 friction 7 gravity 8 inertia 9 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) 10 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: 11 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) 12 13 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)