Section 7.3 Law Of Conservation Of Momentum

Section 7.3 Law of conservation of momentum

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7.3 Law of conservation of momentum (SB p. 165)

Law of conservation of momentum a net force

change in momentum What is the change in momentum during a collision?

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Activity 1 © Manhattan Press (H.K.) Ltd.

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7.3 Law of conservation of momentum (SB p. 165)

Law of conservation of momentum

Before collision

During collision

After collision

Before collision: velocity of A = uA

After collision: velocity of A = vA

velocity of B = uB

velocity of B = vB

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7.3 Law of conservation of momentum (SB p. 165)

By Newton’s third law, Impulsive force on A by B (FA) = − Impulsive force on B by A (FB) Rate of change of momentum of A = − Rate of change of momentum of B Change of momentum of A = − Change of momentum of B of impact of B of impact of A = − ChangeTime ChangeTime of momentum of momentum

mA× vA − mA× uA = − (mB× vB − mB× uB) mA× vA + mB× vB = mA× uA + mB× uB Total momentum after collision = Total momentum before collision © Manhattan Press (H.K.) Ltd.

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7.3 Law of conservation of momentum (SB p. 165)

Law of conservation of momentum

Law of conservation of momentum: → no external forces → total momentum before collision = total momentum after collision mAuA + mBuB = mAvA + mBvB © Manhattan Press (H.K.) Ltd.

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To section 7.4

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7.3 Law of conservation of momentum (SB p. 164)

Activity 1: Newton’s cradle

Let’s start: 1. Pull a ball to a certain height. Then releases it. Ans What happens? wer

When a ball is released and hit the other balls, a ball on the other side swings up to about the same height. © Manhattan Press (H.K.) Ltd.

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7.3 Law of conservation of momentum (SB p. 164)

Activity 1(Cont): Newton’s cradle

2. Pull two balls to a certain height. Then releases them. What happens? Repeat the experiment with three and then four balls. Ans What happen? wer When two balls are released and hit the other balls, two balls on the other side swing up to about the same height. When three or four balls are released, the same number of balls on teh other side swing up. © Manhattan Press (H.K.) Ltd.

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7.3 Law of conservation of momentum (SB p. 164)

Activity 1(Cont): Newton’s cradle

3. Can you explain the above results from the law of conservation of Ans momentum ? wer

From the law of conservation of momentum, the momentum before collision is equal to the momentum after collision. Therefore, when one or more balls are released, an equal number of balls on the other side swing up. Return to © Manhattan Press (H.K.) Ltd.

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