How Everything Works   Page 17 of 160 (1595 Questions and Answers) Printer Friendly Version

161. Why do the front brakes of a bike provide more braking power than the rear brakes, assuming both are applied with equal pressure?
When you apply brakes on a bicycle, you make it harder for the wheels to turn. The ground must then exert backward frictional forces on the wheels to keep them turning and it is these backward frictional forces that slow the bicycle's forward motion. But the forces that the ground exerts on the bottoms of the wheels also produces a torque on the bicycle about its center of mass—the whole bicycle has a tendency to begin rotating. Fortunately, the bicycle rarely actually rotates—if it did, you would fly forward over the front wheel of the bicycle. But this tendency to rotate during braking pushes the bicycle's front wheel downward onto the pavement and lifts the bicycle's back wheel upward off the pavement. The added pressure between the front wheel and the pavement improves traction there and makes the front wheel particularly effective for braking. The loss of pressure between the back wheel and the pavement reduces traction there and makes the back wheel particularly ineffective for braking. In fact, it's easy to begin fishtailing as the rear wheel loses traction completely.

162. Why is it easier for you to make sharp turns more quickly when your center of gravity is over the handle bar?
The force that causes you to turn is friction between the front wheel and the ground. When you turn left, friction pushes the front wheel left and you turn left. By putting all of your weight over the front wheel, you accomplish two things. First, you increase the maximum static frictional force between the ground and the front wheel. You push them together harder so that they are less likely to slide (skid). Second, you make it easier for that sideways friction force to accelerate you; the force acts closer to your body and more directly on you. There are fewer torques on the bicycle that might cause it to skid about on either the front or rear wheel.

163. How does a jackhammer work?
A jackhammer (or pneumatic hammer) uses compressed air to drive a metal piston up and down inside a cylinder. Each time the piston nears the top of the cylinder, it opens a valve that allows compressed air to flow above it and push it downward. Each time the piston reaches the bottom of the cylinder, it opens a valve that allows compressed air to flow below it and push it upward. Thus the compressed air makes the piston shuttle up and down very rapidly.

But while the piston rebounds gently from a cushion of air at the top of the cylinder, it collides suddenly with a metal bar at the bottom of the cylinder. That metal bar is the top end of the drill bit that the jackhammer uses to cut into pavement. Each time the piston moves downward, it pounds the drill bit a little farther into the pavement. The enormous force that pushes the bit into cement comes from the enormous force needed to stop the descending piston and to accelerate it upward. The drill bit pushes up on the piston very hard and the piston pushes down on the drill bit very hard. These two forces are equal and opposite, as they must be (Newton's third law of motion.) The piston ends up moving upward and the drill bit ends up moving downward.

164. If you pulled on the bottom of a multiple pulley while an object was hanging from the end of the multiple pulley's rope, would that object feel heavier than it really is?
Yes. If there were 5 segments in the multiple pulley, then you would have to pull down on the bottom of the multiple pulley with a force that was 5 times the magnitude of the object's weight in order to lift the object at constant velocity. But the object would also rise 5 times as fast as the end of the multiple pulley would descend.

165. What is the difference between a multiple pulley system in which the string you pull on comes down from the top pulley and the one in which the string you pull on comes up from the bottom pulley?
When the string you pull on comes down from the top pulley, it doesn't exert its tension on the thing being lifted so it doesn't count when add up the strings. But when the string you pull on comes up from the bottom pulley, that string is also helping to lift the object. That string does count. Thus if the multiple pulley has 5 segments going up and down between the two pulleys and one more segment going up to your hand, the total number of segments lifting the object is 6 and that object experiences an upward force equal to 6 times the tension in the string.

166. Why do many buses use air brakes instead of hydraulic brakes?
As you have noticed, buses, trucks, and trains often use air as the hydraulic fluid in their braking systems. That's because air is cheap and non-toxic, so that spilling it isn't a problem. While air's compressibility makes it a bit more complicated to work with than a liquid hydraulic fluid, it still works well in power braking systems.

167. With a pulley of 5 strings, why is each string experiencing 10 N of force and not 2 N apiece (when you pull on the string with 10 N of force)?
When you pull on the string with a 10 N force, you create 10 N of tension in that string. If there is less tension anywhere in the string, then that portion of the string will accelerate toward the side with more tension. That's why the tension in each string of a multiple pulley is 10 N when you pull on its loose end with a force of 10 N. The 5 strings are really just parts of the same string and that string has to have 10 N of tension in it.

168. Can a flame occur in space outside a spacecraft, where there is no oxygen? Can it burn or explode there?
For a piece of fuel to burn, it needs a source of oxygen. In open space, there is no oxygen and thus no way for fuel alone to burn. However, materials such as gunpowder that contain both a fuel and an oxidizing agent can burn in open space. In fact, because they don't rely on convection to bring new oxygen into the flame and to carry the burned gases away from the flame, such materials burn almost exactly the same way in space as they do on earth.

169. How does a convection oven work? How is it different from a regular oven?
In an electric convection oven, a fan circulates air rapidly through the cooking chamber. This rapid force circulation of air has two principal effects. First, it ensures that the temperatures throughout the oven are almost exactly equal. In a normal electric oven, the differences in temperatures that often occur lead to uneven cooking and require that you put the food in specific areas of those ovens to make sure that the food cooks properly. Since a convection oven has no temperature differences, you can put the food anywhere and you can fill the cooking chamber more completely with food. Second, a convection oven transfers heat more evenly to the food. By blowing hot air past the food, the oven prevents regions of colder air from building up near the surfaces of cool foods. Since the food in a convection oven is always in contact with hot air, it picks up heat faster and cooks faster. In a normal oven, heat is transferred to the food through normal convection (rising hot air and sinking cold air) and by radiation (particularly when the broiler is used). Both of these process are relatively slow and can be interrupted by over-filling the oven or blocking the line of sight between the hot filament and the colder food. In a convection oven, heat is transferred to the food mostly by forced convection (fan-driven hot air that circulates rapidly through the oven). This process is relatively fast and can't be interrupted by over-filling the oven (within reason) or blocking any line of sight between the hot filament and the food.

170. How does a steam heating system work?
A home steam heating system consists of a boiler, pipes, and radiators. The boiler is located in the basement and uses a burning fuel or electricity to heat water until it boils. Steam forms as the water boils and this steam accumulates above the liquid water. Steam isn't the mist that forms above a teapot—that's really just droplets of water. Steam itself is the clear gaseous form of water and it travels upward through the pipes to radiators in the rooms. Steam is actually a lighter-than-air gas and it's lifted upward by the same buoyant force that makes helium float. When the steam arrives inside the radiators, it begins to condense back into liquid water. As it does so, it releases an enormous amount of heat—the water molecules begin to stick to one another and they release chemical potential energy. After a short time, the temperature of the radiator rises until a balance is reached where the steam and the water are in equilibrium—typically about 100° Celsius, but dependent on the gas pressure inside the radiator. The hot radiator then heats the room. The water that forms as the steam condenses is carried by gravity back down the same pipe through which the steam arrived and returns to the boiler to be reheated.

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