Newtons 3 rd law and momentum
description
Transcript of Newtons 3 rd law and momentum
![Page 1: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/1.jpg)
Newtons 3rd law and momentum
![Page 2: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/2.jpg)
![Page 3: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/3.jpg)
• 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…
![Page 4: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/4.jpg)
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!!
![Page 5: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/5.jpg)
• Calculate momentum two ways:
1) Actual momentum at one time
2) Change in momentum between start and end times
![Page 6: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/6.jpg)
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
![Page 7: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/7.jpg)
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
A B
![Page 8: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/8.jpg)
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.
![Page 9: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/9.jpg)
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)
21m/s15m/s1800kg 1500kg
![Page 10: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/10.jpg)
2D change in momentum
• Change in momentum in 2 or 3D needs vectors.
change = final vector – initial vector
![Page 11: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/11.jpg)
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
0.7
0.8
3.5m/s
0.8
0.7
2.8m/s
![Page 12: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/12.jpg)
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
2.24kgm/s
2.8kgm/s
3.6kgm/s on an angle of 37o
![Page 13: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/13.jpg)
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
![Page 14: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/14.jpg)
![Page 15: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/15.jpg)
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”
![Page 16: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/16.jpg)
• 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.
![Page 17: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/17.jpg)
I’LL PULL HIM
devishlyclever
WELL, ACTION FORCE AND REACTION FORCE ARE ALWAYS
EQUAL AND OPPOSITE!!
![Page 18: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/18.jpg)
WELL ACTION AND REACTION ARE ALWAYS EQUAL AND OPPOSITE!!
Action force and reaction force are always equal and opposite,
SO WHY DOES THE GIRL MOVE FASTER?
![Page 19: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/19.jpg)
NEWTON’S THIRD LAW PAIRS
• THEY ARE EQUAL IN MAGNITUDE
• THEY ARE OPPOSITE IN DIRECTION
• THEY ACT ON DIFFERENT BODIES
![Page 20: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/20.jpg)
The 2 forces act along the same line
SIMILARITIES
The 2 forces act for the same length of time
The 2 forces are the same size
Both forces are of the same type
DIFFERENCES
The 2 forces act on different bodies
The 2 forces are in opposite directions
NEWTON’S THIRD LAW PAIRS
![Page 21: Newtons 3 rd law and momentum](https://reader035.fdocuments.net/reader035/viewer/2022062221/5681406a550346895dabe5c2/html5/thumbnails/21.jpg)
THE CLUB EXERTS A FORCE F ON THE BALL
FF
THE BALL EXERTS AN EQUAL AND OPPOSITE FORCE F ON THE CLUB