The Open Door Web Site

Mechanics

Motion

Quantity

Definition

Vector/
Scalar

S.I. Unit

Displacement

distance moved in a specified direction

v

m

Speed

distance moved per unit time

s

ms-1

Velocity

distance moved per unit time in a specified direction

v

ms-1

Acceleration

change in velocity per unit time

v

ms-2

The Equations of Motion

These equations are useful for solving problems in which objects are moving with uniform acceleration.

If the sense of the motion changes, decide which sense is positive before starting the calculations.

t  represents time
a  represents acceleration
u  represents "initial" speed (or velocity)
v  represents "final" speed (or velocity)
s  represents the displacement from some reference point (usually the starting point) at time t.
If the reference point is not the starting point, we might also see so in the equations (where so means the displacement at time t = 0).

The equations will give us the magnitude and sense of the velocities, accelerations and displacements.

1. Average velocity,

so
2. Acceleration,
3. Equation 2 is often rearranged to give:
4. Combining equations 1 and 3 in order to eliminate v gives:
5. Combining equations 2 and 4 in order to eliminate t gives:

Displacement against Time Graphs
a) stationary
b) uniform velocity

 
uniform acceleration

The slope of a s/t graph represents velocity.

 

© David Hoult 2008