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
