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Newtons 3 rd law and momentum. S I R I S A A C N E WTON (1647 - 1727). Newton worked in the 1600s. He talked about momentum before he talked about force but, maybe because momentum is hard to conceptualise, we learn Newton’s Laws as statements about force.

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Newtons 3 rd law and momentum

Newtons 3rd law and momentum


Newtons 3 rd law and momentum

S I R I S A A C N E WTON

(1647 - 1727)


Newtons 3 rd law and momentum

  • Newton worked in the 1600s. He talked about momentum before he talked about force but, maybe because momentum is hard to conceptualise, we learn Newton’s Laws as statements about force.

  • Momentum is always conserved. Because momentum is always conserved, pairs of forces must be equal and opposite.

  • Let’s look at momentum…


Momentum and collisions

Momentum and collisions

  • We use momentum to solve collision problems in isolated systems.

  • An isolated system has no external forces eg games of pool, frictionless surface problems

    momentum = mass x velocity

    p = mv

    (kgms-1)=(kg) x (ms-1)

  • Momentum is a vector. Use vector diagrams if story not one dimensional!!


Newtons 3 rd law and momentum

  • Calculate momentum two ways:

  • Actual momentum at one time

  • Change in momentum between start and end times


1 momentum now

1 Momentum now

  • On icy winter roads a 1500kg car is travelling at 21ms-1. Calculate the momentum.

  • P = mv

    = 1500 x 21

    = 31 500 kgms-1


2 changing momentum

Initial momentum of A plus initial momentum of B

mAviA + mBviB

Equals final momentum of A + final momentum of B

mAvfA + mBvfB

2 Changing momentum

A

B


Newtons 3 rd law and momentum

Question:On icy winter roads a 1500kg car travelling at 21m/s collides with a 1800kg van going 15m/s in the opposite direction. The two vehicles lock together (1D collision) and move off with a new speed v. Find v.


Newtons 3 rd law and momentum

Answer:

Draw a diagram

Find the initial momentum of each and add (considering direction)

Find the combined mass and multiply by new v

Equate and solve

Ptruck = 1800 x 15

Pcar = -1500 x 21

Ptotal = 4 500 kgms-1

After

Mass = 3300kg

4 500 = 3300 x v

V = 4500/3300

= 1.4 ms-1 (in direction of car)

1800kg

15m/s

21m/s

1500kg


2d change in momentum

2D change in momentum

  • Change in momentum in 2 or 3D needs vectors.

    change = final vector – initial vector

Use diagrams!


Newtons 3 rd law and momentum

Question:

An ice hockey puck of mass 0.8kg moving at 3.5ms-1 hits the side of a second puck initially at rest. The mass of the second puck is 0.70kg.

After the collision the 0.8kg puck moves off at 2.8ms-1 at right angles to its original direction.

Find the velocity of the second puck immediately after the collision

2.8m/s

0.8

0.8

0.7

0.7

3.5m/s


Answer change in momentum of 0 8 puck equal and opposite to change in momentum of 0 7 puck

Answer:Change in momentum of 0.8 puck equal and opposite tochange in momentum of 0.7 puck.

0.8 puck

Final – initial= Final + opposite

2.8kgm/s

3.6kgm/s on an angle of 37o

2.24kgm/s


So 0 7 puck has equal and opposite change

Initially at rest so momentum = 0

final – initial = 3.6 – 0

v = p /m

= 3.6 / 0.7

= 5.1 m/s on 37o

3.6kgm/s on an angle of 37o

So 0.7 puck has equal and opposite change


Newtons 3 rd law and momentum

NEWTON’S THIRD LAW :

“ACTION AND REACTION ARE ALWAYS EQUAL AND OPPOSITE”

“IF A BODY A EXERTS A FORCE ON BODY B, THEN B EXERTS AN EQUAL AND OPPOSITELY DIRECTED FORCE ON A”


Newtons 3 rd law and momentum

  • Momentum are calculated at one time or over a change in time.

  • Forces re calculated over a change in time. Mathematically, this is in the acceleration number.


Newtons 3 rd law and momentum

devishly

clever

I’LL PULL HIM

WELL, ACTION FORCE AND REACTION FORCE ARE ALWAYS EQUAL AND OPPOSITE!!


Newtons 3 rd law and momentum

Action force and reaction force are always equal and opposite,

WELL ACTION AND REACTION ARE ALWAYS EQUAL AND OPPOSITE!!

SO WHY DOES THE GIRL MOVE FASTER?


Newtons 3 rd law and momentum

NEWTON’S THIRD LAW PAIRS

  • THEY ARE EQUAL IN MAGNITUDE

  • THEY ARE OPPOSITE IN DIRECTION

  • THEY ACT ON DIFFERENT BODIES


Newtons 3 rd law and momentum

NEWTON’S THIRD LAW PAIRS

SIMILARITIES

DIFFERENCES

The 2 forces act for the same length of time

The 2 forces act on different bodies

The 2 forces are in opposite directions

The 2 forces are the same size

The 2 forces act along the same line

Both forces are of the same type


Newtons 3 rd law and momentum

F

F

THE CLUB EXERTS A FORCE F ON THE BALL

THE BALL EXERTS AN EQUAL AND OPPOSITE FORCE F ON THE CLUB


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