Newton's Second Law
Forces and Motion

Acceleration and deceleration

Physics Narrative for 11-14 Supporting Physics Teaching


Please take care with acceleration. This note explains some concerns.

The word acceleration is used to describe motion which changes. It is common to use acceleration to mean speeding up and the word deceleration to mean slowing down. Strictly speaking, however, acceleration describes both types of motion. Speeding up might be referred to as positive acceleration and slowing down as negative acceleration. However, raising this point may confuse pupils at this level, with little addition to their understanding. Later in their studies of science, pupils will be introduced to a more formal definition of acceleration in terms such as:

acceleration = change in speedtime for change

(units of acceleration: metresecond in each second, or metresecond2).

If they are introduced to an even more precise definition, it will be that acceleration is change of velocity each second. Pupils will also measure accelerations and resultant forces and relate them to each other. Sir Isaac Newton, in his second law of motion, saw that the greater the resultant force, the greater will be the resulting acceleration of a fixed mass. This finding is summarised in the relationship:

Fm = a

Where F is force in newton; m is mass in kilogram; a is acceleration in metresecond2.

Newton's Second Law
is expressed by the relation F=ma
can be used to derive Kepler's First Law

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