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1、Newtons Laws of MotionReviewBackgroundSir Isaac Newton (1643-1727) an English scientist and mathematician famous for his discovery of the law of gravity also discovered the three laws of motion. He published them in his book Philosophiae Naturalis Principia Mathematica (mathematic principles of natu
2、ral philosophy) in 1687. Today these laws are known as Newtons Laws of Motion and describe the motion of all objects on the scale we experience in our everyday lives.“If I have ever made any valuable discoveries, it has been owing more to patient attention, than to any other talent.” -Sir Isaac Newt
3、on Newtons Laws of Motion1. An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an unbalanced force. 2. Force equals mass times acceleration (F = ma). 3. For every action there is an equal and opposite reaction.Newtons First LawAn object at re
4、st tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.What does this mean?Basically, an object will “keep doing what it was doing” unless acted on by an unbalanced force.If the object was sitting still, it will remain stationary. If it was
5、moving at a constant velocity, it will keep moving.It takes force to change the motion of an object.What is meant by unbalanced force?If the forces on an object are equal and opposite, they are said to be balanced, and the object experiences no change in motion. If they are not equal and opposite, t
6、hen the forces are unbalanced and the motion of the object changes.Some Examples from Real LifeTwo teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.A soccer ball is sitting at rest. It takes an unb
7、alanced force of a kick to change its motion.Newtons First Law is also called the Law of InertiaInertia: the tendency of an object to resist changes in its state of motionThe First Law states that all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to
8、 change its motion).More Examples from Real LifeA powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force t
9、o stop them.On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you dont even feel it).If objects in motion tend to stay in motion, why dont moving objects keep moving forever?Things d
10、ont keep moving forever because theres almost always an unbalanced force acting upon it.A book sliding across a table slows down and stops because of the force of friction.If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.In outer space, away from grav
11、ity and any sources of friction, a rocket ship launched with a certain speed and direction would keep going in that same direction and at that same speed forever.Newtons Second LawForce equals mass times acceleration.F = maAcceleration: a measurement of how quickly an object is changing speed.What d
12、oes F = ma mean?Force is directly proportional to mass and acceleration. Imagine a ball of a certain mass moving at a certain acceleration. This ball has a certain force. Now imagine we make the ball twice as big (double the mass) but keep the acceleration constant. F = ma says that this new ball ha
13、s twice the force of the old ball.Now imagine the original ball moving at twice the original acceleration. F = ma says that the ball will again have twice the force of the ball at the original acceleration.More about F = maIf you double the mass, you double the force. If you double the acceleration,
14、 you double the force.What if you double the mass and the acceleration?(2m)(2a) = 4FDoubling the mass and the acceleration quadruples the force.So . . . what if you decrease the mass by half? How much force would the object have now?What does F = ma say?F = ma basically means that the force of an ob
15、ject comes from its mass and its acceleration. Something very small (low mass) thats changing speed very quickly (high acceleration), like a bullet, can still have a great force. Something very small changing speed very slowly will have a very weak force.Something very massive (high mass) thats chan
16、ging speed very slowly (low acceleration), like a glacier, can still have great force.Newtons Third LawFor every action there is an equal and opposite reaction.What does this mean?For every force acting on an object, there is an equal force acting in the opposite direction. Right now, gravity is pulling you down in your seat, but Newtons Third Law says your seat is pushing up a
