# Physics Topic 3 and 4 Revision Notes

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Vector Quantities:

• displacement --> the distance travelled in a straight line- it has both a direction and a size.
• velocity --> the speed of an object/the time rate of change in one particular direction
• acceleration -->  is the rate of change of velocity of an object with respect to time
• force -->  a push or pull upon an object resulting from the object's interaction with another object.

Interpretation of Distance-time Graph:

When an object is stationary, the line on the graph is horizontal. When an object is moving at a steady speed, the line on the graph is straight, but sloped.

Note that the steeper the line, the greater the speed of the object. The blue line is steeper than the red because it represents an object moving faster than the one represented by the red line.

The red lines on the graph represent a typical journey where an object returns to the start again. Notice that the line representing the return journey slopes downwards.

Common equation used to calculate speed:

speed                     =            distance     /         time

(metres per second m/s)    (metres, M)   /     (seconds, S)

Common equation used to calculate acceleration:

Interpretation of Velocity-time Graphs:

When an object is moving with a constant velocity, the line on the graph is horizontal. When an object is moving with a constant acceleration, the line on the graph is straight, but sloped. The diagram shows some typical lines on a velocity-time graph.

Velocity - time graph

The steeper the line, the greater the acceleration of the object. The blue line is steeper than the red line because it represents an object with a greater acceleration.

Notice that a line sloping downwards - with a negative gradient - represents an object with a constant deceleration - slowing down.

• compare acceleration from gradients qualitatively
• calculate the acceleration from the gradient (for uniform acceleration only)
• determine the distance travelled using the area between the graph line and the time axis (for uniform acceleration only)

Free body diagram: (draw and interpret):

Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation.These diagrams will be used throughout our study of physics. The size of the arrow in a free-body diagram reflects the magnitude of the force. The direction of the arrow shows the direction that the force is acting. Each force arrow in the diagram is labeled to indicate the exact type of force. It is generally customary in a free-body diagram to represent the object by a box and to draw the force arrow from the center of the box outward in the direction that the force is acting. An example of a free-body diagram is  shown above.

What happens when bodies interact?

the forces they exert on each other are equal in size and