Electrons move from the atoms in the cloth to the atoms in the balloon, causing the balloon to have a negative charge
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Work needed: 720 J
Explanation:
The work needed to stretch a spring is equal to the elastic potential energy stored in the spring when it is stretched, which is given by

where
k is the spring constant
x is the stretching of the spring from the equilibrium position
In this problem, we have
E = 90 J (work done to stretch the spring)
x = 0.2 m (stretching)
Therefore, the spring constant is

Now we can find what is the work done to stretch the spring by an additional 0.4 m, that means to a total displacement of
x = 0.2 + 0.4 = 0.6 m
Substituting,

Therefore, the additional work needed is

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C.) <span>The total mass of an object can be assumed to be focused at one point, which is called its center of "Mass"
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The energy bar eaten by Sheila has chemical energy locked up inside it. This chemical energy is converted to mechanical energy in form of potential and kinetic energy and this in turn is converted to heat energy as the run progresses. Thus, the energy changes are: chemical energy to mechanical energy [kinetic and potential] and finally to heat energy.
Ways to increase friction
<span>- increase the roughness of the contact materials </span>
<span>- increase the pressure on the contact </span>
<span>Ways to decrease friction </span>
<span>- float the moving body on air </span>
<span>- suck out any air </span>