A1 Motion,mass,and forces
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A1 Motion,mass,and forces
Units of measurement • Scientists make measurements using SI units such as the
• Movement of 12m to the right: displacement = +12 • Movement of 12m to the left: disMotion,mass,and forces
Displacement • Displacement is not necessarily the same as distance travelled. For example,
•
12m
•
A
B
• The arrow above represents the displacement of a particle which moves 12m from A to B. However, with horizontal or vertical motion, it is often more convenient to use a ‘+’ or ‘-’ to show the vector direction. For example:
• Average velocity is not necessarily the same as average speed. For example, if a ball is thrown upwards and travels a total distance of 10m before returning to its starting point 2s later, its average speed is 5 m s-1. But its average velocity is zero, because its displacement is zero.
when the ball below has returned to its starting point, its vertical displacement is zero. However, the distance travelled is 10m.
5m
ball thrown up from here
• The SI uint of speed is the metre/second, abbreviated as m s-1. For example, if an object travels 12m in 2s, its average speed is 6m s-1.
• Average velocity is calculated like this:
ball returns to starting point
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A1 Motion,mass,and forces
Speed and velocity
• Average speed is calculated like this:
• • average speed = •
distance travelled time taken
Speed and velocity
6m
• The velocity above is for a particle moving to the right at 6m s-1. However, as with displacement, it is often more convenient to use a ‘+’ or ‘-’ for the vector direction.
• • average speed = •
displacement time taken
• The SI unit of velocity is also the m s-1. But unlike speed, velocity is a vector.
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A1 Motion,mass,and forces
metre,kilogram,second,and newton.These and their abbreviations are covered in detail in B1.However,you may find it easier to appreciate the links between different units after you have studied the whole of section A. • For simplicity,units will be excluded from some stages of the calculations in this book,as in this example: • total length = 2+3 = 5m • Strictly speaking,this should be written: • total length = 2m+3m = 5m
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A1 Motion,mass,and forces
Displacement
• Displacement is distance moved in a particular direction. The SI unit of displacement is the metre (m) .
• Quantities, such as displacement, which have both magnitude (size) and direction, are called vectors.
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A1 Motion,mass,and forces
Acceleration
• Average acceleration is calculated like this: