Equilibrium: As shown in the figure, a woman is straining to lift a large crate, but without success because it is too heavy. We denote the forces on the crate as follows: P is the magnitude of the upward force being exerted on the crate by the person, C is the magnitude of the vertical contact force on the crate by the floor, and W is the weight of the crate. How are the magnitudes of these forces related while the person is trying unsuccessfully to lift the crate?
A. P + C = W
B. P + C < W
C. P + C > W
D. P = C
Answer: A
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A changing current in one loop of wire can induce a current in another loop of wire even though the wires do not touch. The induced current appears almost instantaneously for loops close together. How can this energy exchange happen?
a. This is one of those things for which science has no adequate explanation yet. b. The second loop is affected by the thermal exchange from the heating of the first loop. c. It is a gravitational effect on the electrons. d. It is done via an electromagnetic wave.
Given that all bodies in the solar system should have suffered equally from collisions during the period of early bombardment, if we find a body with relatively few craters on its surface we deduce that
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A room is heated with electric resistance wires embedded in the 2.2 m high ceiling. Estimate the heat transfer per unit of floor area when the ceiling is at 80ºC and the floor is 10ºC. Neglect any radiation and heat transfer between the walls of the room.
What will be an ideal response?
A spaceship leaves Earth at a steady forward speed of 70% the speed of light. How fast relative to the ship must the spaceship launch a shuttlecraft in the forward direction so that it will be moving away from Earth at 98.7% the speed of light?
A) 0.91c B) 0.97c C) 0.93c D) 0.99c E) 0.84c