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Newton's Laws applications

  1. Jul 31, 2010 #1
    Hi there, I have two simple physics questions.

    One day I was on the bus, and began to wonder why I kept getting thrown to the back every time the bus accelerated. Which one of Newton's laws can be applied to this?-if any.

    Another question is: When a rocket takes off, it applies a force in the opposite direction. Can this be described as Newton's 3rd law: F₁=-F₁

    Thanks for the help!
  2. jcsd
  3. Jul 31, 2010 #2

    Doc Al

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    Staff: Mentor

    Newton's 1st law might shed some light. You tend to keep moving at the same speed in a straight line unless a force acts on you. When the bus accelerates, you tend to stay put until something pulls or pushes you along with the bus.

    Yes, Newton's 3rd law applies. The rocket exerts a force on the exhaust gases it expels, which in turn exert an equal force on the rocket.
  4. Aug 6, 2010 #3
    Could Newton's third law be applied to the bus? The bus exerts a force forward and I fly back?
  5. Aug 7, 2010 #4
    1.newtons first law 2.yes
  6. Aug 7, 2010 #5


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    Gold Member

    Not until you hit the back of the seat. When the bus accelerates, it doesn't apply a force to you. You simply crash into it. However, when you do hit the seat, you will apply a force against the bus and the bus will exert a force on you, but not before that.

    Suppose you and the bus are at rest at 0 m/s and you are sitting forward in your seat. When the bus accelerates, it begins to have a positive velocity while you still have 0 velocity since the force of the tires and engine are acting on the bus but not you. Soon after, the bus seat will hit you. At that point, the bus acts with a force on you that makes you accelerate until your velocity matches the bus velocity. Due to Newton's 3rd law, you also exert a force on the seat in the other direction decreasing the acceleration of the bus. However, since the bus is powered by a giant engine and you are very small compared to the bus, the force acting on the bus due to you hitting it is not noticable to the bus.
    Last edited: Aug 7, 2010
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