Answer:
- Direct current is a current in which electrons flow in one direction only
- Alternating current is a current in which the direction of the electron flow reverses periodically - so, half a cycle forward, half a cycle backward
There are several advantages of using alternating currents for the transmission of electricity across a country, over large distances. The main advantages are:
- The voltage of alternating currents can be easily increased/decreased by using transformers. For instance, a transformer is used at the beginning of the transmission line to increase the voltage (electricity is transmitted at high voltage in order to reduce dissipated power), and then another transformer is used before the electricity enters the houses, in order to decrease the voltage. Transformers only work with alternating currents.
- It is easy to interrupt the flow of an alternating current, because its value naturally becomes zero every half a cycle, so this is useful in case the current must be interrupted.
Models are very common. The ingredients list on a bottle of ketchup is a model of its contents, and margarine is a model of butter. A box score from a baseball game is a model of the actual event. A trial over an automobile accident is a model of the actual accident.
Answer:
The second system must be set in motion 
seconds later
Explanation:
The oscillation time, T, for a mass, m, attached to spring with Hooke's constant, k, is:
One oscillation takes T secondes, and that is equivalent to a 2π phase. Then, a difference phase of π/2=2π/4, is equivalent to a time t=T/4.
If the phase difference π/2 of the second system relative to the first oscillator. The second system must be set in motion 
seconds later
Answer:
2632 foot-pound
Explanation:
Work done: Work is said to be done when ever a force moves a body through a given distance. The S.I unit of force is Newton (N).
From the question,
The expression for work done is given as,
W = Fdcos∅......................... Equation 1
Where W = work done, F = force, d = distance, ∅ = angle between the force and the horizontal.
Given: F = 32 lbs, d = 90 feet, ∅ = 24°
substitute into equation 1
W = 32×90×cos24
W = 2880(0.914)
W = 2632.32
W = 2632 foot-pound
