PRESSURE Chapter 3: Section 1 Pressure Describes how a force is spread over an area. Describes how a force is spread over an area. It is a measure of how much force is acting on a certain area. It is a measure of how much force is acting on a certain area. Pressure: A measurement of toughness? I need a volunteer I need a volunteer Pressure from your pets When a cat lies down on your lap, the force of the cats weight is spread out over a large area of your lap. When a cat lies down on your lap, the force of the cats weight is spread out over a large area of your lap. If the cat stands up, all of the force from the cats weight is concentrated into its paws. If the cat stands up, all of the force from the cats weight is concentrated into its paws. Increasing Pressure You can increase pressure in two ways: 1.Increase the force 2.Decrease the area of the force This is a Fact! This is why a person wearing a high-heeled shoe exerts more pressure on the ground than an elephant does. An elephant weighs a lot more than a person, but its feet have more surface area than the pointed heel of a shoe. This is why a person wearing a high-heeled shoe exerts more pressure on the ground than an elephant does. An elephant weighs a lot more than a person, but its feet have more surface area than the pointed heel of a shoe. Pressure & Survival Snow Shoes Vs. Sneakers Snow Shoes Vs. Sneakers Bear Grylls using physics to survive! Bear Grylls using physics to survive! Bear Grylls using physics to survive! Bear Grylls using physics to survive! Demonstration time! How much mass do you think it will take to pop the balloon on the bed of nails? How much mass do you think it will take to pop the balloon on the bed of nails? Let me explain If you were to step on the point of one nail it would most likely go through your foot. If you were to step on the point of one nail it would most likely go through your foot. On the other hand, if you were to step on hundreds of pointy nails, you would not have 100 holes in your foot. Why is this? On the other hand, if you were to step on hundreds of pointy nails, you would not have 100 holes in your foot. Why is this? Put simply, 100s of nails would be supporting your weight instead of just 1 nail. (more surface area which means less pressure exerted on your foot per nail) Put simply, 100s of nails would be supporting your weight instead of just 1 nail. (more surface area which means less pressure exerted on your foot per nail) The Pressure Equation Pressure = Force / Area Pressure = Force / Area P = F / A P = F / A Units: N/m 2 or 1 Pascal (Pa) Units: N/m 2 or 1 Pascal (Pa) Pascal is the base unit for pressure. Pascal is the base unit for pressure. The Pressure Triangle P F A Problems with Pressure What is the pressure exerted by a book that has an area of 0.2m 2 and a weight of 10 N? What is the pressure exerted by a book that has an area of 0.2m 2 and a weight of 10 N? P=F/A P=F/A P=10N/0.2m 2 P=10N/0.2m 2 P=50 N/m 2 or 50 Pa P=50 N/m 2 or 50 Pa Pressure acts in all directions in fluids Fluids are made of loosely connected particles that are too small to see. Fluids are made of loosely connected particles that are too small to see. Their motion is random; they crash into one another and into anything around. Their motion is random; they crash into one another and into anything around. As they collide with an object in the fluid, they apply constant pressure to the surfaces of the object. Misconception Alert! Pressure calculations will not always involve the force of a fluid. Pressure calculations will not always involve the force of a fluid. Since weight is a measurement of gravity, anything that has weight exerts pressure. Since weight is a measurement of gravity, anything that has weight exerts pressure. Example: A crate on a floor exerts a pressure on the floor. Example: A crate on a floor exerts a pressure on the floor. How to calculate weight? Weight is measured in Newtons Weight is measured in Newtons Since weight is a measurement of the force of gravity this should make sense Since weight is a measurement of the force of gravity this should make sense If you know an objects mass in kg, you can calculate that objects weight. If you know an objects mass in kg, you can calculate that objects weight. The equations: w=m*g The equations: w=m*g w= weightm=mass (in kg) g=9.8 N/kg w= weightm=mass (in kg) g=9.8 N/kg Lets see an Example What is the weight of a car if it has a mass of 1000kg? What is the weight of a car if it has a mass of 1000kg? w=m*g w=m*g w=1000 kg * 9.8 N/kg w=1000 kg * 9.8 N/kg w=9,800 N w=9,800 N Pressure in Air At sea level the pressure of the air in the atmosphere exerts 2.2 lbs. of pressure on every square centimeter of your body. At sea level the pressure of the air in the atmosphere exerts 2.2 lbs. of pressure on every square centimeter of your body. You dont notice it, though, because your body provides an equal reaction force! (This should remind you of a law we talked about!!!!) You dont notice it, though, because your body provides an equal reaction force! (This should remind you of a law we talked about!!!!) This is a fact! The air in a large room in your house weighs about as much as an average adult male! (736N) approx. 165 pounds The air in a large room in your house weighs about as much as an average adult male! (736N) approx. 165 pounds Pressure in Air (cont.) Changing Elevation- As you go up, air pressure decreases. Changing Elevation- As you go up, air pressure decreases. Changing Density- Air at lower elevations is denser. Changing Density- Air at lower elevations is denser. As you go up the air pressure on you decreases. Your ears dont adjust to the pressure change very quickly. You may feel pain or experience popping until your eardrum adjusts. Pressure in Water Water exerts more pressure on you than air because it is denser. Water exerts more pressure on you than air because it is denser. As you go deeper, the pressure increases. As you go deeper, the pressure increases. At 3300 ft. the pressure of the water would collapse your lungs if you werent wearing special deep sea diving equipment. At 3300 ft. the pressure of the water would collapse your lungs if you werent wearing special deep sea diving equipment. Mythbusters explore pressure Talk about pressure Talk about pressure Talk about pressure Talk about pressure Summary Pressure is a measure of how much force is acting on a certain area. Pressure is a measure of how much force is acting on a certain area. You can increase pressure in two ways: You can increase pressure in two ways: 1. Increase the force 2. Decrease the area of the force P = F / A P = F / A Pressure acts in all directions in fluids. Pressure acts in all directions in fluids. As elevation increases air pressure decreases. As elevation increases air pressure decreases. As depth in water decreases pressure increases. As depth in water decreases pressure increases. Now its your turn Name 3 times you experience pressure in your everyday lives Name 3 times you experience pressure in your everyday lives