Chapter 4 Forces Flashcards
Newton 1st law
A body will remain at rest or continue at uniform velocity unless acted on upon by a RESULTANT FORCE
Newton 2nd law ?
Equation
Full version?
Acceleration experienced by a body is DIRECTLY PROPORTIONAL to the resultant force acting upon it
- F=MA
- F = change in momentum
Newton 3rd law
When two bodies interact they exert forces upon each other which are equal in magnitude but opposite in direction
And act on OPPOSITE BODIES , so it doesn’t cancel out
Resultant force give rise to what?
What does resultant force up mean
Only acceleration , without it motion can still happen also resultant force up
Means deceleration not flying up remember
What is acceleration under free fall?
Normally just g
But with drag g- D/m
What is mass
Link to weight ?
How measure weight ?
A measure of a body’s inertia - its resistance to a change in velocity
Mass will always be constant at normal speeds however weight Is product of gravitational force times mass
3) use Newton meter which shows 9.8 n when 1 kg attached
what happens in elevator
come back
what is the centre of mass definition proper both
a point through where any externally applied force produces straight line motion but no rotation and
where the body’s mass can be thought to be concentrated, weight vertically down
define where the centre of mass is for a uniform mass distribution object
the point at which all lines of symmetry cross
how to find the centre of mass Sarai answer
1) suspend a body using a clamp or something near its edge with holes
ALLOW IT TO COME TO REST, so its in equilibrium and weight acts through COM
2) suspend a plumb line which will always be straight due to gravity
3) draw a line vertically down using the plumb line
4) now repeat for a different edge until lines of symmetry intersect.
how to trial and error to find com with ruler
what about in space?
balance card on ruler until balanced and draw line, repeat until lines intersect
2) in space as no gravity have to apply force through which will make STRAIGHT LINE motion but no rotational , imagine line and repeat another orientation where they intersect minor
what is a drag force
what are the two factors most important in calculating drag
a force that opposes the motion of the object in fluids
- speed and cross sectional area (viscosity and density of fluids)
drag is proportional to v^2
and drag is proportional to cross sectional are
what is freefall and what is equation that links everything together
freefall is the motion of a body through a drag producing medium without a drag producing device
here a =f/m +g-d/m = g -d/m
here an increase of mass for the same drag will thus increase the maximum acceleration and thus final velocity too
what is freefall and what is equation that links everything together
freefall is the motion of a body through a drag producing medium without a drag producing device
here a =f/m +g-d/m = g -d/m
here an increase of mass for the same drag will thus increase the maximum acceleration and thus final velocity too
how do you reach terminal velocity and how explain how something with more mass but same drag reaches faster
1) at point of drop there is no drag so resultant force is just weight and accelerate down at 9.8
2) as speed increase, so does drag, therefore acceleration decreases as resultant force decreases
3) at one point the drag = weight to there is no resultant force, and as no speed increase drag is constant. this means constant velocity- terminal
4) if parachute open then drag > weight so resultant force and declaration, but this cause drop in speed which cause drop in drag until they are same again, so terminal velocity
for someone with less drag in same mass or more mass in same drag, they will have a greater acceleration and so go at a faster terminal velocity
when can dragn be ignored and when not
- can be because its not comparable to weight several mag lower
however
- if very fast then drag increase
- or mass very small to surface area in which drag to mass ratio insane and max acceleration low
What is moment and principle of moment
Force time perpendicular distance
For a body in rotational equilibrium the sum of the anti-clockwise moments = clockwise about any point
What conditions needed for equilibrium
Net force acting on it is 0, net moment about any point is also 0
Where does support force act
Always at the pivot or point of contact with a surface
How to make an object ROTATE WITHOUT TRANSLATION, what’s is this
Learn
Here you apply a pair of equal forces but in the SAME PLANE but opposing directions and parallel
This is a couple
What is a couple
Torque
A pair of EQUAL forces that give rise to resultant MOMENT but no RESULTANT FORCE , thus no translational movement ( along a distance)
Moment of the couple is known as torque
Coplanar
Act in same plane and meet up
Can never be greater than the addition of three or less than the subtraction of them
Triangle of forces
When can use
Only equilibrium
All three head to toe
How to find density
Mass
Displacment can let it drop to zero and measure
Pressure
Force /area NORMAL FORCE
We normally add up all normal force to give you one
Density of mixtures ?, what assumptions you need to make here .
That they don’t react when combining which don’t affect there density of packed molecules
Pressure in a liquid =
Pressure = depth * density of fluid * gravational pull
How to derive the pressure in fluids equation
At the bottom of a column Pressure will still be force / area
Force = mass * g
Mass = density * volume
Volume = height * cross a
As a result mass = density * height * cross a
So force = g * density * height * cross a
And if you cancel a you get g * density * height
What is the pressure in the same fluid in the same depth no matter what the cross sectional
As g constant and h contestant and p constant = the same
How to do change in pressure in same fluid
Pressure change = change in height * density * gravity
How to do total pressure at depth
REMEMBER TO ADD ATMOSPHERIC
How is upthrust about from change in pressure and how is it equal to weight of fluid displaced ???? Important
Upthrust is a force and due to the pressure differences at top and bottom of a object , as p=f/a so f= p*a
- different pressures due to depth means different forces and resultant upward is upthrust
Heres if cross section Sam’s then
Upthrust = change in pressure * area
= change in height g density * area
2) change in height * area = volume
Volume * density = mass
Mass * g = weight of fluid
= upthrust = weight of fluid displaced
This what is equation for upthrust
Change in height * density * gravity * cross sectional area too
And what principe have you described and what is it ? (Arch)
(Archimedes principle ) = the upthrust experienced by an objects immersed in a fluid whether fully or partially submerged is EQUAL to the WEIGHT of the fluid dispersed
So what determines if object floats or sinks
For an object to float its upthrust must = it’s weight
Which means it’s weight must = the weight of fluid displaced
If it can’t displace a weight = to its weight , then it will sink until it can
If it still can’t it will sink all the way ( this because of densities and how that relates to weight )
How to work out how much somethint will be displaced if they give you densities
Divide little by big trust
A few things to note about weight and displacement , is volume constant ?
Weight of somethint displaced won’t necessarily mean same weight but volume will always be constant
This if you displace weight 0.2 and that is = to whole object inside then that is = to all of its volume !
How to show fraction of cube under water = d1/d2
What important
Basically say volume is x3 and then you know weight as it’s volume* density * gravity
This has to be equal to weight of water displaced , same volume x3 but actually a fraction of volume so fx3 multiplied by d2 and g
And rearrange for the fraction which is d1/d2!!!
Important = find the weights !!
