P4 Flashcards
Centre of mass
Imaginary point where entire weight of an object can be seen to act
centre of mass = centre of gravity (in a uniform gravity field)
the point where any force has no turning effect on the body
may be outside of the object
how to find centre of mass
attach clamp and pin to clamp stand then hang plumb from a string which will become vertically hung when at rest
make small holes in an object then hang from pin - draw a line where plumb overlaps object (vertically), then repeat, and CoM is where lines overlap
What determines stability of object
width of base and height of CoM above surface
high width and low Com optimal
What determines if an object falls
If CoM is unsupported i.e will fall alone the whole object falls
Forces on a slope
Normal force perpendicular to slope and up
Weight straight down
make triangle with W - W cos θ = normal force and W sin θ is force down the slope
θ is angle of slope made with horizontal (from the ground)
calculate friction from given value of speed (or none - if so then = W sin θ)
W replaceable with mg
factors affecting drag
mainly speed and csa (csa has square factor impact)
roughness, texture, density have some impact too
how terminal velocity works
starts to fall - initially no drag - force = weight so accelerates starting at 9.81, as speed increases drag increases then acceleration decreases until it reaches 0, terminal velocity where drag = weight so speed constant
velocity time graph gets a plateauing curve
How to investigate motion in fluid
attach one light ball to a pulley on the other side is an object falling in fluid, the movement of the ball will mirror the movement of the object so can be used to find velocity and other values
Moments
moment = Fd
d must be perpendicular so may need resolving by trigonometry
producing turning forces
Principle of moments
For a body in equilibrium, sum of clockwise = sum of anticlockwise moments
Couples
two forces parallel along different lines in opposite directions causing turning
Torque of a couple
F x d/2 + F x d/2
i.e Fd where d is distance between two forces and F is one of the forces
Triangle of forces
If you have 3 coplanar forces use triangle forces to resolve - when a triangle closes the resultant force is 0
or resolve each bit and make equations
Density and pressure
density mass per unit volume - p = m/V
SI unit kgm-3
pressure normal force per csa - p = F/A
Si unit = Nm-2 or pascal
liquid pressure and derivation
p = hpg - h is depth, g gravity, p density
pressure i.e force of water acting on you (works in all directions)
so mass x g
so pvg
so pahg then divide by csa
so pahg/a = phg or hpg
Upthrust
upward force caused by differing pressures on the bottom and top of an object
force at top = hpgA, force at bottom = (h+x)pga so upthrust = xpga
Archimedes principle
upthrust = weight of fluid displaced
Pressure of an object at surface level water
not 0 but just normal atmospheric pressure (1x10^5 Pa)
