Newton's Laws and Linear Momentum
Newton's laws form the foundation of how we understand motion. Newton's first law tells us that objects at rest stay at rest, and moving objects keep moving at constant speed unless something pushes or pulls them - think of a football rolling until friction stops it.
Newton's third law states that forces always come in pairs - when you push on something, it pushes back with equal strength. These action-reaction pairs are always equal and opposite, act along the same line, involve the same type of force, but affect different objects.
Linear momentum measures how hard it is to stop a moving object. The formula is simple: momentum = mass × velocity. A heavy lorry moving slowly can have the same momentum as a light car moving quickly, which is why both can cause similar damage in crashes.
💡 Quick tip: In a closed system (no external forces like friction), momentum is always conserved - the total momentum before a collision equals the total momentum after!
