1. Imagine a bunch of pixies trapped in an airtight jar. This jar is then placed
ID: 2095374 • Letter: 1
Question
1. Imagine a bunch of pixies trapped in an airtight jar. This jar is then placed on a scale. Would the
scale measure the largest weight when the pixies are flying around in the jar, or when the pixies are
sitting on the bottom of the jar? Explain.
2. Consider a horse-drawn carriage. Through Newton's Laws we know that when the horse pulls on
the carriage, the carriage pulls back on the horse with an equal and opposite force. Explain why we are
still able to observe the horse and carriage moving forward.
3. When astronauts are in the international space station they are still orbiting Earth, and therefor
clearly remain within the gravitational field of Earth. Explain why astronauts appear weightless in the
previously described situation. Hint: Think about how the astronauts are moving relative to the Earth
and consider their orbital path.
Be sure to complete all parts of the 3 questions above.
Explanation / Answer
2)
When you want to apply Newton's 3rd Law, you need to consider the entire system (i.e The horse and the cart). In the case of the system, When the horse pulls on the cart, the cart pulls back on the horse. However, when we want to determine the resultant action (or force) we consider isolated objects. That is why we draw free body diagrams on isolated objects and not on an entire system.
Hence, in the case given above, the free body diagram of the cart will be:
(force acting on horse by cart)<------C
and the free body diagram of the horse will be:
(force exerted by horse to move forward)<------------H------>(force acting on cart by horse)
You realise that the resultant for both objects is to the left. So the resultant action is to the left, and both objects move.
3)
An orbiting spacecraft or space station continues in orbit because its forward momentum carries it around the planet even as gravity tries to pull it down. So astronauts are literally "falling" toward the Earth when they are in orbit. This "freefall" means that they do not experience the actual acceleration force. Nothing in an orbiting spacecraft experiences effective gravity, so moving around is much easier, and objects have to be secured to prevent them from floating away.
However, objects still have the samemass, so will have the same inertia if moving: an iron weight thrown across the cabin will impact with the same force if it hits something, and a huge satellite in a shuttle launch bay will still require a lot of energy to push from the bay. But the absence of gravity would make exercises such as a one-hand push-up ridiculously easy to do.
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