Question

***** No calculations involved, just explain in words. 2-3 sentences are enough.

1. Serious bicyclists say that if you reduce the weight of a bike, it is better to reduce it in the wheels rather than in the frame. Why would reducing weight in the wheels make it easier on the bicyclists than reducing the same amount on the frame?

2. Why is a tapered water glass with a narrow base easier to tip over than a glass with straight sides? Does it matter if the glass is full of empty?

3. Using Bernoulli's theorem, explain how a plane can take off?

***** No calculations involved, just explain in words. 2-3 sentences are enough.

Answer 1

1. The wheels, not the bicycle itself, rotate. In order to get the wheels to rotate (and thus get the bicycle to start moving), a torque must be applied to them. Reducing the weight of the wheels reduces the necessary magnitude of this torque. It is in this application that the wheels are differentiated from the rest of the bicycle

2. A glass tips when its center of gravity is pushed beyone the edge of its base. The answer depends on the center of gravity. For a given base, the glass is easier to tip the higher the center of gravity. For a center of gravity at a given height, the more narrow the base the easier it is to tip over. If the water lowers the center of gravity it makes it harder to tip over the glass. If the water increase the center of gravity it makes the glass easier to tip. Also, the weight of the water makes it take more force to top over the glass.

3. The aerodynamic lift on the wing of an airplane (airfoil) is generally explained by the argument that the faster speed of the air along the top of the wing leads to reduced air pressure there and hence produces a lift.