<span>When a person lifts the block, the block has more potential energy. Therefore the person does positive work on the block.
work = m g h
work = (4.5 kg) (9.80 m/s^2) (1.2 m)
work = 52.92 joules
The person's work on the block is 52.92 joules
When the block is being raised, the force of gravity opposes the motion. Therefore the force of gravity does negative work on the block.
work = - (force) (h)
work = - m g h
work = -(4.5 kg) (9.80 m/s^2) (1.2 m)
work = -52.92 joules
The work done by the force of gravity on the block is -52.92 joules
Note that when the block is moved horizontally, the potential energy does not change. Therefore there is no work done on the block when it moves horizontally (we are assuming that the kinetic energy does not change).</span>
The transit method requires watching the light output of a star over long periods of time. A transit occurs when the planet crosses in front of its star from earths point of view. Since there is a small object (the planet) now blocking some of the star, it appears to dim a little bit for a while until the planet passes. If we are in a position where that occurs regularly (most paths of planets do not happen to be on the line of sight between earth and their star) we can deduce the period of orbit. From the amount of dimming and the period you can estimate the mass
V= s/t = 400/4.5=800/9 (km/h)
12344
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The question in choice-C is the correct answer to your question.
(Is this confusing ?)
