Forces & Motion
Newton's laws, speed, velocity, acceleration and momentum
Speed = distance ÷ time. Velocity = displacement ÷ time (includes direction). Acceleration = change in velocity ÷ time.
Distance-time graphs: gradient = speed. Horizontal line = stationary. Steeper line = faster speed.
Velocity-time graphs: gradient = acceleration. Area under graph = distance travelled. Horizontal line = constant velocity.
Newton's Laws of Motion:
- First Law (Inertia): An object stays at rest or constant velocity unless acted on by a resultant force.
- Second Law: F = ma (Force = mass × acceleration). Larger force → more acceleration. Larger mass → less acceleration.
- Third Law: Every action has an equal and opposite reaction.
Momentum** = mass × velocity (kg m/s). **Conservation of momentum: Total momentum before = total momentum after (in a closed system).
Friction is a force opposing motion. Weight = mass × g (gravitational field strength; g = 10 N/kg on Earth).
Stopping distance = thinking distance + braking distance. Increases with speed, tiredness, wet roads.
Key Points to Remember
- 1Speed = distance ÷ time; acceleration = Δv ÷ t
- 2F = ma (Newton's Second Law)
- 3Momentum = mass × velocity; conserved in collisions
- 4Stopping distance = thinking + braking distance
Pakistan Example
Rickshaw Braking in Karachi Traffic — Forces in Action
A loaded rickshaw (mass 400 kg) travelling at 10 m/s applies brakes with 800 N force. Deceleration = F/m = 800/400 = 2 m/s². Stopping distance depends on speed squared — doubling speed quadruples braking distance. This explains why accidents are so severe at high speeds on Karachi's M9 motorway.
Quick Revision Infographic
Physics — Quick Revision
Forces & Motion
Key Concepts
Formulas to Know
v ÷ tF = ma (Newton's Second Law)Momentum = mass × velocity; conserved in collisionsStopping distance = thinking + braking distanceRickshaw Braking in Karachi Traffic — Forces in Action
A loaded rickshaw (mass 400 kg) travelling at 10 m/s applies brakes with 800 N force. Deceleration = F/m = 800/400 = 2 m/s². Stopping distance depends on speed squared — doubling speed quadruples braking distance. This explains why accidents are so severe at high speeds on Karachi's M9 motorway.