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DEMO: Speed and velocity

Worked examples

• A runner covers $100$ m in $25$ s: $v = 100 \div 25 = 4$ m/s.
• A bus at $8$ m/s for $12$ s: $d = 8 \times 12 = 96$ m.
• $36$ km/h $= 36 \div 3.6 = 10$ m/s.
• $500$ m at $20$ m/s: $t = 500 \div 20 = 25$ s.
• $6$ m east then $8$ m north: $\sqrt{6^2 + 8^2} = \sqrt{100} = 10$ m.
• A walker goes $100$ m out and $100$ m back in $50$ s: displacement $0$ m, so velocity $0$ m/s (average speed $200 \div 50 = 4$ m/s).

Key points

• Speed = distance ÷ time.
• Divide $150$ m by $30$ s.
• Distance = speed × time.
• Multiply $20$ m/s by $15$ s.
• $1$ km/h $= \frac{1000}{3600}$ m/s $= \frac{1}{3.6}$ m/s.
• Divide $72$ by $3.6$.
• Time = distance ÷ speed.
• Divide $600$ m by $12$ m/s.
• Displacement is the straight line from start to end, not the path length.
• The two legs are at right angles: use $\sqrt{3^2 + 4^2}$.
• Velocity uses displacement (start to end), not distance.
• Start and end are the same point.

Teacher video

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