SME
Grade 8 Science · Problem type
Balanced or unbalanced forces?
Use this for “What causes the sliding rock to slow down?”
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Are the forces on it balanced?
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Assumptions
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Balanced does not mean stopped: a steady speed in a straight line has no net force.
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Slowing down means an unbalanced force, usually friction, against the motion.
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Related lessons
Newton's three laws of motion
Main lesson
Two skaters push apart: force pairs
Use this for “Two skaters push off each other. Who moves, and who speeds up more?”
Net force: a pull against friction
Use this for “A 50 N pull against 30 N of friction on an 8 kg box. What is the acceleration?” and “Why must you keep pushing to keep a box sliding at a steady speed?”