Exploration Series. TIRE PRESSURE Interactive Physics Simulation Page 01

Transcription

1 TIRE PRESSURE Interactive Physics Simulation Page 01

2 How can air hold up a truck? Over-inflating or under-inflating your tires can waste energy and even cause a rupture. How much air is the right amount? Let s investigate the science behind this. Air pressure in the tire is generated by the collisions between the air molecules and the interior tire wall. Adding more air increases the number of collisions, and therefore the air pressure. When the air pressure increases, the area of contact between the tire and the ground shrinks in proportion. The product of air pressure and contact area is applied force. Each tire applies a force to the ground due to the air pressure within. The third law pair force applied by the ground to the tire is what supports the car. To access this physics simulation visit: Page 02

3 Vehicle Mass- This slider adjusts the mass of the vehicle. The total force required to support the vehicle against gravity will be roughly 10 times this value (in Newtons) since g = 10 N/kg approximates the Earth's gravitational field. Each tire, of course, only supports some fraction of the total weight - to see the force each tire supports, you'd divide the total weight of the truck by the number of tires. Gauge Pressure - This slider adjusts the gauge pressure in the tire. Gauge pressure refers to the pressure in the tire ABOVE atmospheric pressure (1 atm). So, if the gauge pressure is 1 atm, that means the absolute pressure in the tire is 2 atm. If the gauge pressure reads 4 atm, that means the absolute pressure in the tire is 5 atm. You increase the gauge pressure by adding more air particles, increasing the temperature of the air, or decreasing its volume. Number of Tires - This slider adjusts the number of tires on the vehicle. Let's say the vehicle has mass 6000 kg, and so has a weight of roughly N. If it has 4 tires, then each tire supports N / 4 = N. To access this physics simulation visit: Page 03

4 Gauge Pressure Vs. Tire Contact Area - This is a plot of the pressure vs. contact area for the tire in question. The contact area is small if the tire is overinflated, and large if the tire is underinflated. The area of the graph shown here represents the force exerted by the air in the tire on the contact patch between the tire and the ground. Through an argument based in Newton's 3rd Law, we can see that this equals the upward force on the wheel as a whole. To convert this area to Newtons, you'll need to convert the pressure to Pascals. 1 Pa = 1 N/m 2, and 1 atm = 101,325 Pa. To access this physics simulation visit: Page 04

5 If the vehicle mass is 1500 kg, what combination of gauge pressure and number of tires will result in an applied force of 2480 N on each tire? (Set the vehicle mass slider to 1500 kg. Adjust the gauge pressure slider and number of tires slider so that the area bounded by the bar graph is 2480 N.) If the gauge pressure is 5 atm, what combination of vehicle mass and number of tires will result in an applied force of 600 N on each tire? (Set the gauge pressure slider to 5 atm. Adjust the vehicle mass slider and number of tires slider until the area bounded by the bar graph is 600 N.) Adjust the sliders so that the maximum amount of force is applied to each tire. (Adjust the vehicle mass slider, the gauge pressure slider, and number of tires slider until the area bounded by the bar graph is greater than 11,110 N.) To access this physics simulation visit: Page 05

6 Challenge ME! Why do more massive vehicles require more tires? Why do we use the gauge pressure (the pressure in the tire ABOVE atmospheric pressure) when calculating the force applied to the car? Why might the air pressure required in your tires differ for the front wheels compared to the rear wheels? Need Help? Check out the Tire Pressure Walkthrough video at: To access this physics simulation visit: Page 06

7 How high is the air pressure in this room? You might be surprised to learn that the air pressure acting on your right now is enormous. Each square centimeter of your skin feels approximately 10 N of force - as if a 1 kg object were resting there. We don't normally experience this air pressure in an unbalanced way unless a vacuum is created. Consider what happens when you suck the air out of a plastic bottle - it crumples under air pressure quite dramatically! How deep can I dive in the ocean? The air pressure at sea level, called 1 atmosphere (1 atm), is very high, but our body is accustomed to it. Each 10 m (approx. 33 feet) of depth you achieve when diving, the pressure increases by 1 atm. At a depth of 1 km, the pressure would be 100 times that at the surface. It is a difficult engineering task to design vehicles that can withstand such pressures. Why are nails used in carpentry? The force required to split wood is very high - imagine trying to just rip a piece of wood apart with your hands! By focusing the applied force on a very small area - the tip of a nail - the pressure exerted on the wood is enough to break chemical bonds and split the wood. The nail is held inside the wood by the pressure of the wood, which presses up against the nail. Which would hurt more: an elephant or a high heel? You don't want your foot stepped on - not by an elephant or by a woman in high heels. However, both present very different challenges. The elephant weighs more, but has a larger foot - so the force is spread out. Your foot might be crushed, but perhaps not pierced. The woman weighs less, but for a very narrow heel, the pressure exerted on your foot might be enough to break the skin. Watch out! To access this physics simulation visit: Page 07

8 Physics Concepts Click on the link below to learn more. Pressure and Force - Pascal's Law - To access this physics simulation visit: Page 08