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# Newton's first law of motion

Newton's First Law of Motion, also known as the Law of Inertia, states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This law, originally formulated by Galileo, is fundamental to understanding motion and forces. Created by Sal Khan.

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• my physics textbook says that if a bus is moving then the people sitting or standing in the bus also move in the with the same speed in the same direction .but according to me how can the people also be moving? and further my book states that when the bus stops suddenly , the upper body still continues to move and the lower body tens to move turn with the bus which causes the passengers to fall forward.
but it isn't clear to me that how is the upper body in motion?
can someone explain!<its stated under the examples of inertia>
• The people are at rest but they are in motion with respect to the ground. If the people were not in motion how will they arrive at their destination?

It is your feet which keeps u in contact with the bus ..right. If the bus stops then your feet will also come to rest. but the tendency of inertia the upper body keeps moving to avoid that sudden jerk of coming at rest.
• At when we were talking about the plane example......we said that if the plane was moving or at rest and we were inside it, we wont realize that whether the plane is movinr or is at rest.......However even if the plane was accelerating with we being inside it......wont we still feel that we are at rest? Isn't it that both our velocity and the acceleration w.r.t. plane zero?
• We actually cannot say, because there is no force acting on us. We have the same velocity as the plane, which is our plain of reference. Thus unless we have a window, we cannot say for certain.
• I know this video was posted a while ago and people probably won't notice it but at , Sal corrected himself and said that the directions are changing. How are the directions changing? Is Earth changing direction? Someone, please answer I am genuinely confused on this subject.
• offcourse, earth is chaging direction because earth is continously rotating.
• Does Newton's 1st law also apply to "reading a book in a car?" Like when the car is moving and you're staring at a book that is "still" and you all of sudden get motion sickness?
• This a question, i have been holding for a while. In a Traffic collision, between a car and a truck, which is a way bigger than the car, both will feel the impact in the same way?
• Both will experience the same force. But that force will result in much more acceleration for the car than the truck, because the car's mass is much less.
• If an object is moving with constant velocity, with no unbalanced force applied, i.e. 5 m/s, can I assume that after 2 seconds , or 5 seconds, or 10 seconds, when I measure its velocity, it would still come out to be 5 m/s?
• If there is no net force (same as no unbalanced force) on an object it will have a constant velocity and which means that for as long as there is not net force it will measure the same velocity.
• when is the force of friction maximum at rest or at motion.
• Max friction occurs at rest. When motion begins it turns from static to kinectic which is always less. At max Friction Force = the coefficient of friction x FN
• when the ice melts and starts to slide on the water, what is the unbalanced causing it to move?