Force and Laws of Motion — Quick Revision
Balanced forces do not change the state of motion of an object; unbalanced forces do. Newton's first law states that an object stays at rest or in uniform motion in a straight line unless an unbalanced force acts on it — this property is called inertia, and it depends on mass. The second law says that the rate of change of momentum is proportional to the applied force, which gives F = ma. The third law states that for every action there is an equal and opposite reaction, acting on two different bodies. Momentum p = mv, and in the absence of an external force the total momentum of a system is conserved.
Key Points
- •Balanced forces: no change in motion
- •First law (inertia): rest or uniform motion continues unless acted upon
- •Inertia depends on mass
- •Second law: F = ma
- •Third law: action and reaction are equal, opposite, on different bodies
- •Momentum p = mv (kg·m/s)
- •Conservation: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂