IB Physics Sub-topic A3 Notes

Work

From Topic A.2, you learned about the different types of forces. It may seem obvious that applying any force requires energy, called work. Work (W) is the measure of the energy required to apply a force to move an object in the direction of the force. As such, the formula for this is:

$W = Fscos\theta$

Despite using displacement and force in the formula, work is a scalar quantity. This is because by definition work is only applied in the direction of the force. 

As a result, if the object moves at a right angle to the force, no work has been done, as cos (90°) = 0. As with any energy, work is measured in Joules (J). 

Springs are a special scenario because the work done is not constant. The more a spring is compressed or stretched, the more work is required to compress or stretch it further.

It thus requires a force-distance graph to determine the work, which is the area under the line.

Energy

As mentioned earlier, it takes energy to apply a force. It thus makes that sense that work done is also equal to the amount of energy transferred. Energy (E) is a measure of the amount of work done, measured in joules (J).

Within any physical interaction, the principle of conservation of energy always applies:

1. the total energy of any closed system stays the same.
2. energy is not created or destroyed, it only changes form.

Most reactions do not have a perfect transfer of energy and will have some transfer into heat and sound.