牛顿第一运动定律

今天就是为大家介绍牛顿第一运动定律鸭～(o^^o)

If you learn about the force and the motion of object, you may ask:What is the relationship between force and motion? 

The ancient Greeks also curios about this.So a sientist called Aristotle (384–322 B.C.) came up with the idea >>that a force was required to keep an object moving along a horizontal plane.To Aristotle>>(翻译成人话)>>the natural state of an object was at rest, and a force was believed necessary to keep an object in motion. Furthermore, Aristotle argued, the greater the force on the object, the greater its speed.

But, about 2000 years later, a person standed out, disagreed: he maintained that it is just as natural for an object to be in motion with a constant velocity as it is for it to be at rest.

Yes, that is the person we kown as: Galileo!!!

To push an object across a table at constant speed requires a force from your hand that can balance the force of friction（如下图）. 

When the object moves at constant speed, your pushing force is equal in magnitude to the friction force; but these two forces are in opposite directions, so the net force on the object (the vector sum of the two forces) is zero. 

This is consistent with Galileo’s viewpoint, for the object moves with constant velocity when no net force is exerted on it.

Upon this foundation laid by Galileo, Isaac Newton built his great theory of motion. Newton’s analysis of motion is summarized in his famous “three laws of motion.” In his great work, the Principia (published in 1687), Newton readily acknowledged his debt to Galileo. In fact, Newton’s first law of motion is close to Galileo’s conclusions. It states that：

Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no net force acts on it.

（所以其实牛顿大概就是一个证实加发表结论的人吧）

The tendency of an object to maintain its state of rest or of uniform velocity in a straight line is called inertia. As a result, Newton’s first law is often called the law of inertia.

举个🌰：

A school bus comes to a sudden stop, and all of the backpacks on the floor start to slide forward. What force causes them to do that?

A：It isn’t “force” that does it. By Newton’s first law, the backpacks continue their state of motion, maintaining their velocity. The backpacks slow down if a force is applied, such as friction with the floor.

But，Newton’s first law does not hold in every reference frame. 

For example, if your reference frame is an accelerating car, an object such as a cup resting on the dashboard may begin to move toward you (it stayed at rest as long as the car’s velocity remained constant). The cup accelerated toward you, but neither you nor anything else exerted a force on it in that direction. 

Similarly, in the reference frame of the decelerating bus（就是上面举的例子）there was no force pushing the backpacks forward. In accelerating reference frames, Newton’s first law does not hold. 

Physics is easier in reference frames in which Newton’s first law does hold, and they are called inertial reference frames (the law of inertia is valid in them). 

For most purposes, we usually make the approximation that a reference frame fixed on the Earth is an inertial frame. This is not precisely true, due to the Earth’s rotation, but usually it is close enough.

Any reference frame that moves with constant velocity (say, a car or an air- plane) relative to an inertial frame is also an inertial reference frame.

Reference frames where the law of inertia does not hold, such as the accelerating reference frames discussed above, are called noninertial reference frames. 

How can we be sure a reference frame is inertial or not? By checking to see if Newton’s first law holds. Thus Newton’s first law serves as the definition of inertial reference frames.

好了，今天的分享就到这里了，谢谢阅读～

喜欢请三连（三连是对我最大的支持）～o(^▽^)o