# Newton's Laws and Momentum

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• Created by: Agata
• Created on: 24-02-16 15:03
• Newton's Laws and Momentum
• Newton's First law
• A Force is necessary to change the state of rest or of uniform motion in a straight line of a body.
• For instance: 1) If you are stationary, you will remain stationary unless a resultant (net force acts on you. 2) If you want to change your direction of travel, a resultant force must act on you. 3) If you want to speed up or slow down, a resultant force must act on you.
• BUT: If you are moving with constant velocity, there is ZERO RESULTANT FORCE acting on you.
• Balanced forces
• NO Net force, NO acceleration
• Force causes acceleration
• WORKED EXAMPLE:
• A) A person of 80 kg is sitting in a bullet train travelling on a straight level track at 80 m/s. What vertical support force and what forward force does the train provide for the person?    ANSWER: The weight of the person is mg = 80 x 9.8 = 784 N, so the support force = 784 N upwards (resultant force zero). Zero forward force because velocity is constant.
• B) The train now moves at the same speed onto a track that is still straight but has an upward gradient of 0.25%. What vertical support force and what forward force does the train now provide for the person?
• ANSWER B: The vertical support force must still be 784 N (resultant force zero). Zero horizontal force because velocity is constant. (If you take components along the slope and at right angles to the slope then you get 1.96 N and 783.998 N respectively. Using Pythagoras' Theorem on these values gives the upward force 784 N with zero overall resultant, since this is equal and opposite weight.
• C) Explain why pouring a drink on a train might be difficult when changing direction, but not when travelling with constant velocity.
• ANSWER C: When changing direction there will be an acceleration as the velocity is changing. The cup into which the drink is being poured is accelerating, but the drink in mid-air has nothing to provide horizontal force, so the drink might well slop over the side of the glass.