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# Mechanics Warmups

Physics is not a discipline, but a way of looking at the world. See if you can use your sense of the world to explain everyday phenomena.

# The great outdoors

A magician fills a glass with some water, lays a glossy postcard on top, and then flips the entire assembly while holding the postcard firm to the bottom of the glass. He then removes his hand and the postcard stays in place, keeping the water in the glass.

How is this possible?

Water in a river speeds up as it goes over the top of a waterfall.

What is the primary reason that this happens?

The Earth is protected by a layer of atmosphere that can absorb heat. Does this keep the Earth warmer, cooler, or at the same temperature as it would be without the atmosphere, on average?

You have just started your summer job as a lifeguard at the beach, which has oversized ants leisurely swimming. You are on top of your lifeguard stand and notice that Brilli the ant is making distress signals some distance offshore, and you want to travel to Brilli in the quickest way possible.

Knowing that you travel faster running over sand than you do swimming through water, which of the above best illustrates the quickest path to take?

Assume that the water is still, with no currents.

People are often surprised to find just how much of an iceberg sits below the surface of the water. Suppose that the density of an iceberg is (uniformly) $$0.92 \text{ g/mL}$$, and that of the ocean is $$1.02 \text{ g/mL}$$. Approximately what percentage of the iceberg's volume is hidden below the ocean surface?

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