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Explain positive, negative and zero work. Give one example of each.

Answer
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Hint: Write formula of work W=Fr. Learn dot product of two forces. For positive work, W should be positive so for this find an angle between force and displacement.
Dot product of two component A and B
AB=ABcosθ
Where θ is the angle between two vectors.
Fr=Frcosθ
Find θ for positive W, negative W and zero W.

Complete step by step answer:
The work done by a force on a particle during a displacement is given as
W=Fr
Here, W = work
F= force
r= displacement
Positive work done – The work done is said to be positive when force and displacement are in the same direction.
θ=0Fr=FrcosθFr=Frcos0Fr=FrW=Fr=Fr
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Hence, work is positive.

Zero work – the work done is said to be zero when force and displacement are perpendicular to each other.
θ=90Fr=FrcosθFr=Frcos90Fr=0W=Fr=0
Hence, work is zero
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Negative work done – The work done is said to be negative when force and displacement are in opposite directions.
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θ=180Fr=FrcosθFr=Frcos180Fr=FrW=Fr=Fr
Hence, work is negative.

Note: Work done by friction is always zero because friction force and displacement act in opposite directions. When a spring travels from A to B and from B back to A then work done during the return journey is negative of the work during the onwards journey and the net work done by the spring is zero.
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