The first law of thermodynamics is simply the conservation of energy restated to relate the thermal energy of a system to heat and work. 

This is saying that the energy of the system is not created or destroyed, it is just changing form.  If we combine it with the more traditional conservation law, it says that mechanical energy of a system can change into thermal energy of the system without violating conservation.  Let’s take a closer look at this:

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The skater is going up and down the ramp without losing energy, because the ramp is frictionless. Due to this, he continues to reach the same height on each end of the ramp as his kinetic energy (energy of motion) is converted to gravitational potential energy (energy of position).

While the skater is moving, click the Bar Graph button and answer the following questions.

What happens to the height of the thermal energy bar while the skater is moving?
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It is zero.

How are the kinetic and potential energy bars changing?

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As kinetic becomes shorter, potential becomes taller. They are moving up and down with constant maximum and minimum values.

What happens to the height of the total energy bar while the skater is moving?
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It stays the same.

To investigate the case where friction is involved, click the "Track Friction" button, and add friction by sliding the bar to the right. Now watch the bar graph and answer the same three questions:

What happens to the height of the thermal energy bar while the skater is moving?
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It starts at zero and rises.

How are the kinetic and potential energy bars changing?
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As kinetic becomes shorter, potential becomes taller. They are moving up and down but each time they reach a smaller and smaller maximum value.

What happens to the height of the total energy bar while the skater is moving?
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It stays the same.

In effect, as friction is heating the rail and the skate board wheels, kinetic energy and potential energy are being converted to heat. Once again, the total energy remains constant, but the skater ends up at the low point on the track as all available energy goes into heating the track and the skate board wheels.