There are two types of equilibrium in mechanics.One is called static equilibrium and the other one is called dynamic equilibrium. In both the cases of mechanical equilibrium,the net force acting on the particle is zero.
A body is said to be in dynamic equilibrium if the net force acting on a moving body is zero.There will be no acceleration of the body.The body will continue its uniform motion without change in its direction and speed.
The body is said to be in static equilibrium if the net force acting on a body at rest is zero.As the net force is zero,the body will not undergo motion. It is due to the inertia of the body.
The two equilibrium are the direct consequences of Newton's first law which tells that a body will continue to be at state of rest or uniform motion along a straight line unless and until it is compelled by some external unbalanced force.Hence as long as net force on the body is zero,the body at rest will satisfy static equilibrium.
Out of the four options given in the question only third option is right which tells that a book that has no net force acting on it and sitting on a table is under static equilibrium. If the net force is not zero,the body can not be under static equilibrium.The book resting on a table imparts a force equal to its weight on the table and table in turn gives the normal reaction in vertically upward direction.The gravity pulls the book in vertically downward direction with a force equal to its weigh.Hence the net force is zero.So the table will be at rest.
If the net force is not zero,the body can not be under static equilibrium.
Hence option 3 is right.
A control group in a scientific experiment is a group separated from the rest of the experiment, where the independent variable being tested cannot influence the results. This isolates the independent variable 's effects on the experiment and can help rule out alternative explanations of the experimental results.
Answer:
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If it produces 20J of light energy in a second, then that 20J is the 10% of the supply that becomes useful output.
20 J/s = 10% of Supply
20 J/s = (0.1) x (Supply)
Divide each side by 0.1:
Supply = (20 J/s) / (0.1)
<em>Supply = 200 J/s </em>(200 watts)
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Here's something to think about: What could you do to make the lamp more efficient ? Answer: Use it for a heater !
If you use it for a heater, then the HEAT is the 'useful' part, and the light is the part that you really don't care about. Suddenly ... bada-boom ... the lamp is 90% efficient !
