Newton’s Laws of Motion

 The British scientist Sir Isaac Newton was able to state rules that describe the effects of forces on the motion of objects.  These rules are known as Newton’s laws of motion.  Inertia is the tendency of an object to resist any change in its motion.  If an object is moving, it will have uniform motion. It will keep moving at the same speed and in the same direction unless an unbalanced force acts on it.  The inertia of an object is related to its mass. The greater the mass of an object is, the greater its inertia.

Newton’s 1 st Law  Newton’s first law of motion states that an object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it.  This is sometimes called the law of inertia.  In a car crash, inertia causes an unrestrained passenger to continue moving at the speed of the car before the crash.

Newton’s 2 nd Law  According to Newton’s 2 nd law of motion, the acceleration of an object is in the same direction of the net force on the object and the acceleration can be calculated from the following equation: acceleration(in meters/second 2 ) = net force(newtons) mass(kilograms) a = F/m

 Gravity is one of the four basic forces.  Gravity is an attraction force between any two objects that depends on the masses of the objects and he distance between them.  Earth’s motion around the Sun is affect by the gravitational pulls of the other planets in the solar system.  In the 1840s the most distant planet known was Uranus. The motion of Uranus calculated from the law of universal gravitation disagreed slightly with its observed motion.  Using the law of universal gravitation and Newton’s laws of motion and two astronomers independently calculated the orbit of this planet. Neptune was found in 1846.

 Close to Earth’s surface, the acceleration of a falling object in free fall is 9.8 m/s 2.  By Newton’s 2 nd law of motion, the force of the Earth’s gravity on a falling object is the objects mass times the acceleration of gravity. force of gravity = mass x acceleration of gravity F = mg

 Whether you are standing, jumping or falling, Earth exerts a gravitational force on you. This force is called weight.  The weight of any object on Earth is equal to the force of Earth’s gravity on the object.  Weight is calculated weight(N) = mass(kg) x acceleration of gravity(m/s 2 ) W = mg

 Weight and mass are not the same. Weight is a force and mass is a measure of the amount of matter an object contains.  The weight of the object can change depending on the gravitational force on the object.  Gravity on the moon is one-sixth of that of the gravity on Earth. So one the moon you would weigh less.

Newton’s 3 rd Law  Newton’s 3 rd law of motion describes action-reaction pairs.  When one object exerts a force on a second object, the second one exerts a force on the first that is equal in strength and opposite in direction.  Example: When you jump on a trampoline, you exert a downward force on the trampoline. Simultaneously, the trampoline exerts an equal force upward, sending you into the air.