Newton's Laws M.K.Rolley

Sir Isaac Newton 1643-1727 You might know that he…. made revolutionary discoveries in Physics Mathematics, Optics and Astronomy Invented calculus & the reflecting telescope. produced universal theory of gravitation and theories of motion

Sir Isaac Newton 1643-1727 You might NOT know that he…. Had school reports that described him as ‘idle and inattentive’ Threatened his parents-‘ to burn them and the house over them’ Was at times, a cantankerous and ill tempered recluse rather too sensitive to criticism, particularly from Hooke and Leibnitz Wrote more theology than physics Was for most of his career ‘Master of the Mint’ who pursued millers of coins to the gallows

Newton’s 1st Law An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted on by an outside unbalanced force

What law 1 means…. Objects keep doing what they are doing unless some force (sometimes not easily seen) changes either speed or direction Objects which are changing speed or direction must have a force acting on them Objects with all forces balanced may be moving with constant speed or be stationary A force is not always necessary to keep things moving

Newton’s second law The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object

What the 2nd law means... a F a  1/m a  F/m or F=kma where k is the proportionality constant and if this is 1 or F=ma

This allows us to define the Newton: ‘One Newton is that force that will give a mass of 1 Kg an acceleration of 1m/s2’

Newton’s 3rd Law “For every action, there is an equal and opposite reaction."

What the 3rd law means…. A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.

Examples 1. A cyclist of mass 60kg rides a bike of mass of 20kg. When starting off the cyclist provides a force of 200N What is his acceleration? 2.5m/s2 2. A car of mass 500kg is travelling at 20m/s. The driver sees a red light ahead and slows to a halt in 10s. What force must the brakes provide to do this? 1000N 3. A rocket has a mass of 5000kg. Its engine provide a thrust of 200,000N . What is its acceleration and why is this liable to increase as the rocket progresses into its flight? 40m/s2

Examples 2 4. Calculate (a)the force required to accelerate a sports car of mass 1449 kg which can accelerate from 0-60mph (26.8 m/s) in 3.9s (b) the force acting on its occupants when it hits a brick wall at 60mph and comes to rest in 1sec. 9957N, 39000N 5. A Pellet of mass 0.001kg is fired at a block of wood at a speed of 110 m/s. It embeds itself to a depth of 0.05m.Calculate the impact force 121N 6.An aeroplane of mass 15000kg lands with a velocity of 55m/s. ‘Reverse thrust’ from the engines slows it to a speed of 25m/s in a distance of 240m. What is the size of the reverse thrust supplied by the engines? -1.5x105N