Newton's First Law says that if the resultant force on an object is zero, a stationary object stays stationary, and a moving object keeps moving at the same speed in the same direction, so at the same velocity. When a vehicle travels at a steady speed, the resistive forces balance the driving force. The velocity of an object only changes if a resultant force acts on it. Higher tier only: the tendency of objects to stay at rest or in uniform motion is called inertia.
Newton's Second Law says that the acceleration of an object is proportional to the resultant force on it and inversely proportional to its mass. As an equation, resultant force = mass × acceleration, or F = ma. Force is in newtons (N), mass in kilograms (kg) and acceleration in m/s2. A 1200 kg car accelerating at 2 m/s2 needs a resultant force of 2400 N. Higher tier only: inertial mass is a measure of how difficult it is to change the velocity of an object, and it is defined as the ratio of force over acceleration.
In the required practical, a trolley is pulled along a bench by a falling weight, and light gates or a ticker timer measure its acceleration. To test the effect of force, keep the mass constant by moving masses between the trolley and the weight hanger, and change the force. To test the effect of mass, keep the force constant and change the mass of the trolley. The results show acceleration rising with force and falling with mass.
Newton's Third Law: whenever two objects interact, the forces they exert on each other are equal and opposite. These two forces act on different objects. A book on a table pushes down on the table, and the table pushes up on the book with an equal force. The book stays in equilibrium because the upward force balances its weight, so the resultant force is zero.
