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**Grade 8 Mechanical Systems**

Topic 1: Work Animation

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GRAVITY Gravity constantly exerts a downward force, seeking to crush us under our own weight.

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GRAVITY When we stand, we exert a force upwards, overcoming (resisting) the force of gravity that is pushing downwards on us.

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Walking ≠ Working When we walk, we move horizontally, which is not in the same direction as our upwards force.

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Walking ≠ Working When we move in the same direction as the force we exert on ourselves, then we are doing work.

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Where’s the Work? Only the distance through which we move in the same direction as the applied force is used in the work formula.

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Where’s the Work? Is work being done on the horizontal portions of the walk?

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**Where’s the Work? What about on the ramp?**

Not on the horizontal portion. Only the vertical distance travelled counts for work.

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**Work and Inclined Planes**

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**Work and Inclined Planes**

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**Work and Inclined Planes**

Regardless of how gentle or steep the ramp, the amount of work done on the object is the same as if the object was just lifted straight up against the force of gravity.

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**Work and Inclined Planes**

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**Work and Inclined Planes**

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What About Stairs? When you walk up stairs, work is done. Coming down the stairs, not so much.

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**Ladders are Inclined Planes Too**

When climbing a ladder, the horizontal distance travelled doesn’t count; only the vertical distance is used for work calculations.

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Stairs Walking up stairs requires work for the vertical portion. To walk downstairs requires no work becasue the direction of travel is the opposite of the direction of our upward force. Walking down stairs is “controlled falling”; it has the same net effect as falling, without the injuries.

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The Long Jump The distance that a person can jump is dependent on two factors: the speed at which she travels down the track and the “hang time” she attains in the air. Distance = speed × time.

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The Long Jump “Hang time” is determined by the vertical portion of the jump as well as body positioning. The jumper pulls up her legs so as to prolong the jump and travel further.

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The Long Jump It is only during the vertical portion of the jump that any work in a scientific sense is being done.

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