The particle model represents a moving object as if all its mass were concentrated at a single point.

Position locates an object with respect to a chosen coordinate system. Change in position is called displacement.

Velocity is the rate of change of the position vector $\vec{r}$.

Acceleration is the rate of change of the velocity vector $\vec{v}$.

An object has an acceleration if it

• Changes speed and/or

• Changes direction.

Pictorial Representation

1 Draw a motion diagram.

2 Establish coordinates.

3 Sketch the situation.

4 Define symbols.

5 List knowns.

6 Identify desired unknown.

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For motion along a line:

• Speeding up: $\vec{v}$ and $\vec{a}$ point in the same direction, $v_x$ and $a_x$ have the same sign.

• Slowing down: $\vec{v}$ and $\vec{a}$ point in opposite directions, $v_x$ and $a_x$ have opposite signs.

• Constant speed: $\vec{a}=\vec{0}\text{,}{a}_x=0$.

Acceleration $a_x$ is positive if $\vec{a}$ ptoints right, negative if $\vec{a}$ points left. The sign of $a_x$ does not imply speeding up or slowing down.

Significant figures are reliably known digits. The number of significant figures for:

• Multiplication, division, powers is set by the value with the fewest significant figures.

• Addition, subtraction is set by the value with the smallest number of decimal places.

The appropriate number of significant figures in a calculation is determined by the data provided.