Conservation of energy Name d. In summary, based on work and energy consideratio

Question

Conservation of energy Name d. In summary, based on work and energy considerations: i. the effect the Earth has on the energy of system s (ie when the Earth is part of the system) ii. the effect the Earth has on the energy of system T (ie.. when the Earth is not part of the 2. A student observes an apple falling from a tree and makes the following statement: The Earth exerts a force on the apple, the force of gravity. The Earth does work on the apple. I can write the following equation to describe this interaction: The work done by the Earth increases the apple's kinetic energy and decreases its potential energy." What mistake is this student making? How would you correct the student's equation? 3. A spring is used to launch a block up a frictionless ramp as shown at Just before right. At time ti, the spring is compressed and the block is released t from point A. The block moves up the ramp, losing contact with the spring at point B. When it passes point C, the block has a velocity vc directed up the ramp. Imagine a system that includes the block and possibly one or more additional objects. a. Is it possible to choose a system whose total energy is constant during the interval from to th? so, state which object(s) must be included in the system. Explain. b. possible to choose a system so that the total energy is not constant the interval from t to If so, what is the system containing the fewest possible objects for which the energy is not constant during this interval? Explain. H W

Explanation / Answer

2. Work done is given as :

Work done = force x displacement in the direction of force

on the apple , earth exert force gravity force ''mg'' in down direction

as the apple falls from tree , displacement ''d'' of the apple is in down direction.

So work done by earth = mg x d = Potential energy of the apple.

The mistake student is making is to include the Kinetic energy.in the work done.

3. a) yes it is possible to choose a system whose total energy is constant. that system includes the spring and the Block. So Spring-Block system has Constant energy.

at t1 , when the spring is compressed. The Total energy of the system is stored as the spring Potential energy

at t2 , the spring is loose and does not have spring potential energy but the block has no gained some height and hence Potential energy which is exactly equal to the spring Potential energy at t1

c) if we choose the system only includes block or the spring alone, then the Total energy of the system at t1 and t2 is not constant.

When spring is the system :

at t1 , Spring has spring Potential energy But t2 , the spring has no Energy. So energy is not constant.

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