Chaptwr 4 Flashcards
What is Newton’s first law
A bidy will remain at rest or continue at uniform velocity unless acted upon by a RESULTANT FORCE
Newton’s second law
Acceleration experience by an object is DIRECTLY proportional to resultant force acting upon it
Forces is directly proportional to rate of change of momentum
Newton’s third law
When two bodies interact, they will exert forces equal in mangnitude but opposite in direction and on opposing bodies
What is mass
What is inertial
How to measure Weight
Mass is a measure if a body’s inertia = hiw hard it is to change a body’s velocity l
Use a newtonmeter which shows 9.8n when 1kg is attached
Weight si the product of the gravational field strength multiple by constant mass
What is the centre of mass of an object? (2)
What else is special about centre of mass
The point where it can be thought that all the weight of the object acts through, concentrated, vertically down
The point at which if an external force is applied through the centre of mass, the object will move with no rotation simply translational movement , straight line motion
What is uniform mass distribution?
The fact that mass is not ckncentrated more on ines idde on an object, sleazed evenly
Assuming uniform mass distribution, where is the centre of mass
The centre of mass is the point where ALL lines of SYMMETRY CROSS
How to find the centre of mass using a plump line?
First explain THEORY
An object suspended freely will have its centre of mass directly below its suspension vertically
1) thus add holes to ends of an object corner
2) suspend the object using a clamp and clamp stand
3) attachach a plumb line and a plumb bib
4) use plumb line to draw a line which the centre of gravity will be on
5) rotate and do again
6) the centre of mass will be the point at which ALL LINES OF SYMMETERY CRISS
How to find com in space?
Push an object until it only moves with straight line motion and no rotation, and imagine a line through it
Now move object and do from another angle
The com will be where these lines intersect
If you want to try and trial and error using ruler, balance card on ruler until it is balanced and imagine line here, rotate do again, com will be where thesenlijesninrersecn
What is any drag force?
Two most important fsctord in calsucktimgdrag
Drag is any force that opposes the ,Orion of an object in fluid
1) as drag is proportional to v2, the speed
2) drag is dependent on cross sectional area, sknthisn
Whatare other factors that affect drag on the side
Density if object, roughness , texture m shale etc
Again what is free fall if an object
This is when an object is accedletaed inky by gravity and does not produce any drag
Explain what happens when you drop an object and drag acts on it
Basically explain the journey ti terminak vekociyt
What about PARACHUTE?
At t= 0, by dropping the object no drag acts on it , thus accerlation = resultant force which is just its weight (/m)
As it picks up speed, it dies drag at exponential rate. As a result resultant force decreases, so acceleration decreases too
Eventually the drag has increases so much such that it is = to its own weight. At this point resultant force has decreased all the way to 0, so there IS NO MORE ACCELERATION . As a result, it now goes at a constant speed = THE TERMINAK VEKOCITY
2) if parachute open, then drag > weight, thus resultant force means deceleration . This decreases speed which decreases drag until drag = weight again, and you are terminak velocity but a lower one now!
Explain how something that has MORE MASS but the same SA as anither object (and thus drag) has a higher terminak velocity then?
Not the sarai fraud
Essentially if it has more mass then more weight, and for it to revsh terminak velocity a greater amount if drag must be acting upon it, and for the same SA it means to reach a higher drag it needs to have a higher velocity too, as v 2is proportional to drag . As a result to reach terminak velocity the velocity of the heavier jbject is higher!
Last time when can drag be ignored
1) if several orders of magnitude less, then ignore = negligible
2) however if mass very small compared to SA
Or at very fast speed
Then Cant
How to investigate drag?
What is the MOMENT OF A FORCE
the moment if a force is the force (acting through the axis) multiplied by the perpendicular distance of the line of action of the force to the axis
What is the principle of moments
What does it mean to be in equilibrium
For a body in ROTATIONAL EQUILIBRIUM, the sum of clockwise moments = the sum of anti-clockwise moments
Equilibrium is where there is no net moment ABOUT ANY POINT and also no net force acting on ANY PLANE . Thus means itsin rotational equilibrium (won’t move anywhere = basically just equilibrium )
Remmeber little things about moments questions
There will normally be a weight of the beam at the centre of mass, I guess assume uniform mass distribution so half way point
- if you have two unknowns, then take moments from one point, as the moment if this will now be 0 due to no perpendicular distance. Then find the other one
Remember if itnis in rotational equilibrium sum of anticlockwise moments = sum of clockwise moments, but also no net force about any plane, can help you make more equations
Finally the support force acts on any point that touches something, the NORMA! So this includes the PIVOT, or like a SURFACE
How do you make an object ROTATE WITHIUT ANY TRANSLATIONAL KOVEMENT?
You apply a pair of equal but opposite forces along the objects , which are parallel and along different lines
This means net force is STILL 0 so no translational movement, however net moment isn’t 0, so there is rotational movement
This is known as a couple
And the moment if a couple is known as torque
What is the torque of a couple then
It is again a PAIR OF EQUAL BUT OPPSITE FIRCES APPLIED IN THE SAME PLANE but oppsiiding directions that are parallel
Calculated by force x Perpednciular distance between separation
Again they give rise to resultant moment but no resultant force, hence only rotational movmentn
What is moment of couple about any point however?
THE EXACT same , terms cancel, = force x perpendicular distance between speartook
What does doplanar mean,
You know how there is different planes , the. All are on one plane but in different directions
What important abotu triangle of foces
That they are HEAD TO TOE
- AND RESULTANT IS JUST 0!,
Remember relative size of the length of the arrow supposed to represent the size of the force
How can you resolve multiple vectors knowing resultant is 0 without even using triangle if foces
Just split into planes, horizontal and vertical planes resolved soearye,y and this way you know
What else about any three coplanar forces resultant?
I PORTSMT
Can never be greater than the addition of all three or less than thr subtraction if all three!!!!
How to measure density
Mass high a weighing scale
Volume using now either change in volume of a measuring cylinder or a eureka can which measured the volume of water displaced
What is pressure, importsnt
Pressure = NORMAL FORCE/ AREA
this is because all normal forces are added up to give a whole valuen
How to calculate density if mixtures?
What assumption ti make A
As this would change perhaps mass/ volumes
Here you do find spearates mass and volumes and do one claultion
You must assume that there is no chemical reaction and rearrangement if atoms when these two mixtures are our totgehe
Pressure in water =?
Does container shape affect pressure?
What is it
DENSITY x Gravity x Height
No the only variable that affects pressure is height , where gravity and density is constant
What is Archimedes principle
The uothrust acting on an object is = to the weight of fluid displaced
Why?
As upthrust = d x g x h x a
Uothrust = mg of fluid
So uppthusy = weight if fluid displaced
What does pressure equation show
It not dependent in cross sectional area nothing , just height
This pressure proportional to height
Hat is change of pressure in a fluid
Just the change of height times density and gravity
How to do total pressure
Total pressure = ATMOSPHERIC + SEA PRESSURE
What even is upthrust
Upthrust is a force , kinda like reaction force caused by the change in pressure resulting ijaforce as an ibjectmoves down
Essentially it is the same as the change in oressure x A
And we know change in oressure = change in height x g x density
So uothrust = a x change in height x g x density
Which is equal to the WEIGHT OF DLUID DISPLACED
Again uothrust is
Is caused by the pressure differences resulting in a dowanrds force from the top and the bottom surface of a container
This resolutes innuothust
Hat determines if object floats or sinks?
An object will only float if it can displace enough fluid to = its own weight, so uothrust = weight and resultant force is 0
It may have to only displace a bit if fludinto do this thus it sinks a bit
However if fully submerged it still can’t (due to density if liquid) then it will sink !
Remmeber what to know about displacement
The volume of water displaced is always = to the volume of the object in water
However the weight if the object displaced is only equal to the weight of upthrust and if itnis floating then the weight ofnthe object too!
How to find out how much an kbkecr will be underwater to float?
Just divide the little density by the big density to get fraction
Show how the fraction of something underwater is = d1/d2
Weight of ibject to float= weight of fluid dispersed
Weight = mg , d= m/v, m=D1V1
D1V1g= weight fluid dispersed
Mg = d=m/v, m= d x v, here the d= d2, and v is the fraction of object under x total volume = FXv1
Rearrange and you get f= d1/d2!!
In all density equation what to remember r
Weights of uothust = weight fluid disperced
If float then weight of ibject = uothrust p
Vilume of the objects is constant and fluid dispersed!
