# First up in Newton’s first law of motion:A

First lawFriction and air resistance cause a car to come to rest when the engine is switched off.If these forces were absent we belive that a body,once set in motion,would go on moving forever with a constant speed in a straight lie.  That is ,force is needed to keep a body moving with uniform velocity provided that no opposing forces act on it.This idea was proposed by Galileo and is summed up in Newton’s first law of motion:A body  stays at rest ,or moving it continues to move with uniformly velocity ,unless an external force makes it behave differently.It seems that the questionwe should ask about a moving body is not “what keeps it moving” but “what changes or stops its motion”. The smaller the external forces opposing a moving body, the smaller is the force needed to keep it moving with uniform velocity.An ‘airboard’,which is supported by a cushion of air , can skim across the ground with little frictional opposition, so that relatively little powe is needed to maintain motion.• Newton’s first law is another way of saying that all matter has a built-in opposition to being moved if it is at rest or, if it is moving,to having its motion changed. This property off matter is called inertia.Its effect is evident on the occupants of a car that stops suddenly; they lurch forwards an an attempt to  continue moving,and this is why seat belts are needed . The reluctance of a atationary object to move can be shown by placinga large coin  on a piece of cardon your finger . if the card is flicked sharply the coin stays where it is while the card flies off . The larger the mass of the body , the greater is its inertie , i.e the more difficult it is move it when at rest and to stop it when in motion . Because of this we consider that the mass of a body measures its inertia. This is better definition of mass than the one given earlier in which it was stated to be’amount of matter’in a body.Second lawNewton’s second law of motion.when using it two points should be noted. First F is the resultant force causing the acceleration a. Second,F must be in newtons , m in kg and a in meters per second squared , otherwise k is not 1 . The law shows that a will be largest when F is large and m is small .  You should know appreciate that when the forces acting on a body do not balance there is a net force which causes a change of motion , i.e the body accelerates or decelerates . If the forces balance, there is no change in the motion of the body . However, there may be  a change of shape , in which case internal forces in the body balance the external forces.Third lawIf a body A exerts a force on body B,then body B exerts an equal but opposite force on body A.This is Newton’s third law of motion and states that forces never occur singly but always in pairs as a result of the acion between two bodies . For example , when you step forwards from rest your footpushes backwards on the Earth , and the Earth exerts an equal and opposite force forward on you. Two bodies and two forces are involved. The small force you exerts on the large mass of the Earth but the equal force it exerts on you very much smaller mass auses you to accelerate .  Note that the pair of equal and opposite forces do not act on the same body ; if they did, there could never be any resultant forces and acceleration would be impossible . for a book resting on a tble, and the table exerts an equal and opposite upward force on the book; this pair of forces act on different objects.The weight of the book does not form a pair with the upward force on the bok as these two forces act on the same body .   An appreciation of the third law and the effect of friction is desirable when stepping from a rowing boat. You push backwards on the boat and, although the boat pushes you forwards with an equal force, it is itself now moving backwards . this reduces your forwards motion by the same amount so you may fall in !