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DEMO: Newton's second law

Worked examples

• $5$ kg at $2$ m/s²: $F = 5 \times 2 = 10$ N.
• $30$ N on $6$ kg: $a = 30 \div 6 = 5$ m/s².
• $24$ N gives $8$ m/s²: $m = 24 \div 8 = 3$ kg.
• $12$ N on $4$ kg gives $3$ m/s²; on $6$ kg it gives $12 \div 6 = 2$ m/s².
• A $3$ kg book: $W = 3 \times 10 = 30$ N.
• A $3$ kg crate: push $20$ N, friction $8$ N. Net $= 12$ N, $a = 12 \div 3 = 4$ m/s².

Key points

• Newton's second law: $F = ma$.
• Multiply $4$ kg by $3$ m/s².
• Rearrange $F = ma$ to $a = F \div m$.
• Divide $20$ N by $5$ kg.
• Rearrange $F = ma$ to $m = F \div a$.
• Divide $30$ N by $6$ m/s².
• For the same force, a larger mass gives a smaller acceleration.
• $a = F \div m = 10 \div 5$.
• Weight is the force of gravity: $W = mg$.
• Multiply $10$ kg by $10$ m/s².
• Find the net force first: push minus friction.
• Net force $= 15 - 5 = 10$ N, then $a = F \div m$.

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