5 Newton’s Laws of Motion Dynamics, the study of how forces influence motion, is involved in understanding a wide range of phenomena. Newton’s first law of motion: An object at rest will remain at rest; an object in motion will continue in motion with constant velocity, except acted upon by an external force. This tells us that force is the cause of motion. The SI unit of force is in newtons. Newton’s second law of motion: The change of momentum per second is proportional to the applied force and the molife mentum change takes place in the direction of the force. F = mv−mv0 t = momentum change time F =ma, (23) (24) where a is acceleration of the particle. The force of gravity acting on an object is called the weight of the object. It can be written as W =mg, (25) where W is the weight in newtons, m is mass in kilogram, and g is acceleration due to gravity in m/s2. Momentum is the product of mass and velocity. It is a vector quantity and its SI unit is kgm/s. The impulse (Force × time) is the ’time effect’ of a force on an object. Its SI unit is Ns. Newton’s third law of motion: To every action, there is an equal and opposite reaction. This means that if a body A exerts a force (action) on a body B; then B will exert an equal and opposite force (reaction) on A .