7TH GRADE PRACTICE
Physics Problems: Work, power and energy, Grade 7
Work, simple mechanisms, efficiency and energy conversion.
Here the conditions and analysis of the solution can be read without an account. In the simulator, you need to collect the same analyzes yourself: “Given,” formulas, order of steps and answer.
Problem statements
The answer options are there to check against, and the walkthrough opens under each problem.
Problem #49
Mechanical work and powerEvaluate the statement: A force does mechanical work when a body moves and the force has a component along the displacement.
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For a constant force along the motion, the work is equal to the product of the force and the path; without movement, mechanical work is zero.
Answer
Problem #50
Mechanical work and powerEvaluate the statement: The concept of “Power” means: Work is positive when a force is applied to movement and negative if the force is directed against movement.
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Two mechanisms can do the same work, but the more powerful one will do it in less time.
Answer
Problem #51
Work and powerA winch pulls a load with a constant force of 300 N over 4 m in 20 s. Find work and power.
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Answer
Problem #52
Levers and blocksEvaluate the statement: A rigid body capable of rotating around a support and transmitting forces with different arms.
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A long arm allows a smaller force to balance a larger force on a short arm.
Answer
Problem #53
Levers and blocksEvaluate the statement: the concept “Blocks and the Golden Rule” means: A value equal to the product of a force by its arm and characterizing the rotating effect of the force.
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A stationary block changes the direction of the force, and a movable block in an ideal design reduces the required force by half.
Answer
Problem #54
Simple mechanismsThe 0.4 m arm of the lever is subject to a force of 50 N. What force must be applied to the opposite 0.1 m arm for balance?
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Answer
Problem #55
Efficiency and mechanical energyEvaluate the statement: The ratio of useful work to total work expended, usually expressed as a percentage.
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Due to friction and deformation, the efficiency of a real mechanism is less than one hundred percent, but the energy does not disappear, but is dissipated.
Answer
Problem #56
Efficiency and mechanical energyEvaluate the statement: the concept “Potential energy” means: The energy of motion of a body, which increases with mass and especially rapidly with increasing speed.
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A lifted load can do work when lowered, and a compressed spring can do work when returning to its original shape.
Answer
Problem #57
Efficiency and energyThe lift spent 1000 J of work, of which 800 J went to lifting the load. Find the efficiency.
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Answer
The theory behind these problems
Work through the topic if a problem will not give in.
Formulas by topic
Look up symbols and units of measurement in the reference.