Grade 8 Science Chapter 9 Notes Force Force - Anything that causes a change in the motion of an object. - usually a push or a pull. - the unit for force is the Newton (N). Balanced Forces - forces that are equal in strength and opposite to each other in direction. Ex: You are putting a downward force on the chair you are sitting on. The chair in turn is putting an equal upward force on you. That is why you do not go through the chair. Unbalanced Forces - forces that are not equal, this causes a movement in an object. Ex: If the chair did not push upward with the same force, you would break through it. Buoyancy Buoyant Force - also called Buoyancy. The upward force on objects submerged in or floating on fluids. Why do objects float? - buoyant force of the fluid pushes upward on the object. - the force of gravity (or weight) on the object pushes it down into the fluid. - if the buoyant force is greater than the weight of the object, then if floats. - if the weight of the object is greater than the buoyant force, the object will sink. Neutral Buoyancy - an object that floats; it is not sinking or rising, it has neutral buoyancy. This is because the force of gravity and buoyancy are equal.
Examples of Technology based on Buoyancy and Density 1. Hot Air Balloon - the air is heated and become less dense, thus rising. - the hot air helps the buoyant force fight the weight of the object allowing it to rise. 2. Submarines - there is a tank inside which allows water to flow in and out of it. - when engineers let water in, the weight of the submarine overpowers the buoyant force and the sub sinks. - when water is pumped out, the buoyant force overpowers the weight and pushes the sub up. Fish Bladder - fish rise and sink through the water in a similar way to submarines. They have a bladder which they fill with air to rise and release the air to sink. 3. PDF (Life Jacket) - made of material with a very low density. - makes the average density of the jacket and the person lighter than the buoyant force. Average Density - total mass of all substances that make up an object divided by the total volume. Note: Jackets should be thrown out after a period of years because the material begins to break down causing the density to increase. The jacket increases the mass of the person, but only slightly increases the volume, therefore increasing the density. How is it possible for large heavy ships to float? 1. Average Density - Because the is hollow, the air inside the ship makes the average density of the ship lower than the density of the water it is floating in. - If the ship were to get a hole in it, the incoming water would replace the air making the average density greater than the water it is floating in and it will sink. 2. Archimedes' Principle - "The buoyant force acting on an object equals the weight of the fluid displaced by the object." - The bottom of a ship is shaped in such a way to help displace water. - The ship will float if it weighs less than the weight of the water it displaced (i.e., the buoyant force).
Pressure Pressure - the force acting on a certain area of a surface. - the unit for pressure is the pascal (Pa). - Newtons/meter squared You can increase pressure in two ways: 1. Increase the force being applied Ex: Push hand against the wall harder 2. Decrease the area that the force is being applied. Ex: Push pin instead of your hand against the wall with the same force will result in the push pin going into the wall. Atmospheric Pressure - the pressure exerted by the layers surrounding the Earth. - close to the Earth's surface, the entire atmosphere is pressing down on the air compressing it. - at higher altitudes, there is less air pressing down, therefore, higher levels of air are not as compressed as much as lower levels. - as a result, air is less dense at higher altitudes. - this means that there is less atmospheric pressure at high altitudes and greater atmospheric pressure at lower altitudes. - This explains why it is more difficult to breathe at higher altitudes. Calculating Pressure F P A
Example 1: Calculate the pressure on the bottom of an aquarium that has an area of 1.6 m 2, if the tank is filled with water that weighs 10 000N. Example 2: If the water in a glass weighs 4.9N and exerts a pressure of 1700 Pa, what is the area of the bottom of the glass? Example 3: If the air pressure in a nail gun is 517 000 Pa and the area of the piston that drives the nail is 0.0005 m 2, what force does the air in the nail gun exert? Practice questions in textbook: Page 351, #1, 2, 3 Page 352, #1, 2, 3 Page 353, #1, 2, 3
Hydraulic and Pneumatic Systems Blaise Pascal was a 17th century scientist who studied pressure. His findings became known as Pascal's Law. Pascal's Law - Pressure applied to an enclosed fluid is transmitted with equal force throughout the entire container. Ex: Squeezing a tube of toothpaste causes pressure to be exerted throughout the tube, making toothpaste come out through the end. Hydraulics - the study of pressure in liquids Hydraulic Systems - devices that transmit applied force through a liquid to move something else. - a force is applied to an enclosed liquid. - the applied force creates pressure that moves a liquid through tubes, pipes or hoses and causes a motion at the end of a hydraulic system. Ex: Brake lines in a car, dentists chair, Jaws of life, dump truck Pneumatics - the study of pressure in gases Pneumatic Systems - devices that transmit applied force on a gas. - a compressor builds up air pressure by compressing the gas. - when the pressure is released the air particles move apart quickly, creating a strong steady force that can perform powerful tasks. Ex: Jackhammers, air brakes on transport trucks Summary: Hydraulic System Pneumatic System State Liquid Gas Volume Definite Indefinite Pressure Not Compressible Compressible
Temperature, Pressure and Volume of Gases Three main ideas: 1. When the temperature of a gas is constant and you increase the pressure on a gas, its volume will decrease. Ex: This allows us to have compressors. Air from the outside is being forced into a smaller space. 2. When the pressure on a gas is constant and you increase the temperature, the volume of the gas increases. Ex: When the air inside a balloon is warmed the balloon becomes bigger because the air inside takes up more room (increased volume). 3. When the volume of a gas is constant and you increase the temperature of a gas, the pressure increases. Ex: If you heat an aerosol can, the pressure inside the can will increase and it can explode. Summary Temperature Pressure Volume Constant Volume Increase Will increase - Constant Pressure Increase - Will increase Constant Temperature - Increase Will decrease