Newton’s three laws of motion 17/03/13 Newton’s three laws of motion

either of rest, or of moving uniformly forward in a right line.” 17/03/13 force “An impressed force is an action exerted upon a body, in order to change its state, either of rest, or of moving uniformly forward in a right line.” (Newton, Definition IV, The Mathematical Principles of Natural Philosophy) • Is a vector quantity; • its magnitude is measured in newtons (N) with a dynamometer. Examples: • hitting a ball with a bat • force of friction between your sneakers and the floor • gravitational force • pulling with your hand on a fishing line • elastic force of a spring • reaction force of the floor on which you are standing • electric force • magnetic force 2

1st law of motion: law of inertia 17/03/13 1st law of motion: law of inertia Galileo’s thought experiment An object at rest will stay at rest, and an object in motion will continue in uniform linear motion, unless acted upon by an unbalanced force. Inertia is the resistance of any physical object to a change in its state of motion or rest. 3

Newton’s 1st law: an example 17/03/13 Newton’s 1st law: an example When a skydiver falls from a hovering helicopter, as her speed increases, the air resistance on her also increases. Eventually, it is enough to balance her weight, and she gains no more speed. She is at her terminal velocity If she is falling at a steady velocity, the forces on her are balanced: her weight downwards is exactly matched by air resistance upwards. 4

inertial reference frame 17/03/13 inertial reference frame An inertial reference frame is one in which Newton’s law of inertia is valid. An inertial reference frame can be at rest or in uniform linear motion. 5

non-inertial reference frame 17/03/13 non-inertial reference frame In a non-inertial reference frame Newton’s law of inertia is not valid and bodies will change their velocity without being acted on by a force. In a non-inertial reference frame the laws of physics depend upon the acceleration of that frame of reference, and the usual physical forces must be supplemented by fictitious forces (i.e. centrifugal force, Coriolis force, etc.). 6

Newton’s 2nd law of motion 17/03/13 Newton’s 2nd law of motion An unbalanced force acting on a body gives it an acceleration in the direction of the force which is directly proportional to the force itself and inversely proportional to the mass of the body. Newton’s 2nd law enables us to compare the results of the same force exerted on objects of different mass. 7

inertial mass & weight inertial mass weight symbol m definition 17/03/13 inertial mass & weight inertial mass weight symbol m definition is a scalar quantity is a measure of an object’s resistance to changing its state of motion or rest when a force is applied is determined by applying a force to an object and measuring the acceleration that results from that force is a vector quantity acts downwards, towards the centre of the Earth is equal to the gravitational force that the Earth exerts on an object SI unit of measurement kg N

Newton’s 3rd law of motion: action and reaction principle 17/03/13 Newton’s 3rd law of motion: action and reaction principle “To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions.” (Newton, The Mathematical Principles of Natural Philosophy) When two bodies interact, the forces exerted on the bodies by each other are always equal in magnitude and opposite in direction: These are three characteristics of forces: 1. action and reaction forces are opposite in direction; 2. action and reaction forces are equal in magnitude; 3. action and reaction forces act on different bodies. 10

Newton’s laws: an example 17/03/13 Newton’s laws: an example All three Newton’s laws can be illustrated by the example below: When the girl pushes to the right against the wall with a force of magnitude F, Newton's 3rd law states that the wall pushes the girl in the opposite direction with a force of the same magnitude. The force exerted on the wall is balanced by constraint forces, therefore the wall stays at rest, according to Newton's 1st law. The force acting on the girl on the skateboard is unblanced, therefore she accelerates to the left as to Newton's 2nd law. 11