Describing motion


Describing motion

POSTED IN O/A Level PHYSICS Blog, MECHANICS






Motion can be described in multiple ways, using graphs, equations, words and symbols. This topic details some of the ways that you need to know about.


Key terms

Motion can be described using some key terms, symbols and units
Name - symbolDescriptionUnits (standard)
Speed - vSpeed is the rate of travel, it is a scalar quantity so doesn't have a direction (it cannot be negativems-1
Velocity - vVelocity is the vector version of speed, the speed of an object in a given directionms-1
Acceleration - aAcceleration is the rate of change in velocity, therefore it too is a vector. Acceleration describes how many metres per second an object increases its speed each second hence the -2 in the units.ms-2
Displacement - sDisplacement is the vector version of distance, it is the distance something has moved in a certain directionm


Graphs

Graphs can also be used to describe the motion of an object.

Distance time graphs

A distance time graph shows how far an object is away from a given point.
Distance time graph
As distance is a scalar quantity the graph should never have a negative gradient.
The gradient of a point on the graph is the speed of the object at that point.

Displacement-time graphs

A displacement time graph shows how far an object is moving and in what direction.
Displacement time graph
This graph shows how a ball moves when thrown in the air, it moves upwards between coming to a peak and returning to earth as a result of gravity.
The gradient at a point on the graph is the velocity of the object at the point. A positive gradient means the object is moving away from the original point and a negative gradient means the object is moving towards the original point. A gradient of 0 (a straight horizontal line) means the object is not moving.

Velocity-time graphs

Velocity are perhaps the most useful graphs which can be used to describe motion. They show how an objects velocity changes over time:
Velocity time graph
The area under the graph is equal to the displacement (as velocity is a vector) of the object.
The gradient of a point on the graph is equal to the acceleration at that point.

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