Newton Mechanics Flashcards
What are the things we use to model to simplify situiaom
1) model everything as particles, means they have no size and all the force is at a single point
2) light = means no weight and has no effect, so like strings
3) inextensibke means it csn’t extend so it’s the same length at all times, so tension across is the same
4) smooth means no friction
Reaction FIRCE
In maths always perpendicular
Tension
Is force that prevents objects from owing away from each other , so outward and they PUKL
Thrust
Is force which prevents objects from coming together
It can be found in a rod but not in a string or rope (because this wouldn’t be taught enough so like string become floppy)
Arrows inwards and push
Equilibrium
Means resultant force 0 in every plane
And physics it means no reuskktsnt moment
For tension questions etc
Draw the body as one and find all forces acting on it
Remember using second law f =ma, f is the resultant force in that direction put that unknown first
Yh so direction of accerlation, make that the first thing !
Same concept if in vector form!, use variabel wjere you can
Make sure you always draw the direction of acceleration so you don’t mess up which variable to put first
Bro
I guess if it is a force I’m general that pushes something down , you don’t find resultant, because it’s separate?
Just draw a graph for eveything and equate to area it’s easier man
Why is the acceleration of connected objects the same under what assumption?
Under assumption that string is light and inextensibke !
Can combine whole one it’s to one? What happens to tension
It dissapeaes, basically tension is an internal force so is not affected by anything
To find tension in a connected system what to do first
First find acceleration
Now apply to the second law to find out tension
Check it works with the other mass too
Remember rukes for compression (thrust) and type of object
Can only be string rope tension rest is thrust
Okay make sure direction if accerlwtion is correct!
If arrow thenitd notminus! Thus getting wrongs
If you use same direction arrow then Yh minus
So after deceleration if you get a NGETIVE TENSION FIRCE , what does this mean if it is a bar
It means you are pushing towards car so it’s a thrust!
Why are the tensions in a pulley the same?
Because the pulleys are SMOOTJ, so tensions in the strings are the same
Some Importsnt things about pulleys
Here if greater mass on one side, all the others will move in that direction, so that’s direction of acceleration
- the tension AROUND A PULLEY IS THE SAME, the ones on other pulleys are different. Only same because smooth
- if surface is smooth, no friction force!
Find three equations of motion and simultaneously solve
Again importsnt about where the masses go
Even if BIG MASS IN THE MIDDKE
As long as one mass bigger hanging than other side, WHOLE SYSTEM GOES IN THIS DIRECTION !
AND WHAT ABOUT ACCELERATION , is it the same?
Yh if it is caused by the same weight, then all three everything has the SAME ACCELERATION
If it’s just two masses cinncecte by pulley, lit jus pulley in the middle and mass on each side
Okay so for falling down pulley, why can’t you find time and why do you have ti find velocity
Basically time can only be found. Y a quadratic , and so it’s not possible to know for sure which time it was
But velocity is constant
So now basically if it goes 0.2 down it gonna go 0.2 up
And then once the other hits floor thr tension is now 0
So the rising mass resuktsnt firce is only g
So it will decelerate until it reaches 0 velocity and then go downwards again
Now at 0 velocity it means it will be highest height
So need to find the distance it rises from initial velocity (which is same as finak velocity of the other) and to 0
And add to two times 0.2
Giving 0.433
