mechanics Flashcards
define scalar quantity
give three examples
has no direction, only magnitude
mass, length,distance
define vector quantity
give three examples
has both magnitude and size
displacement, velocity, acceleration
describe how you would find the resultant vector of two vectors
scale drawing
tip-to-tail
measure missing side of triangle for resultant vector
describe how you would resolve a vector
Vx = Vcosθ
Vy = Vsinθ
what is a free-body force diagram
shows a single body on its own
shows all the forces acting on the body
arrows
what does it mean to say an object is in equilibrium
all forces acting on it are balanced and cancel each other out
no resultant force on the object
are objects at equilibrium always at rest
no, can be moving at constant velocity too
forces acting on an object in equilibrium have what property when drawn to scale
tip-to-tail
form a closed loop
how would you investigate equilibrium **
using a force board
when ring at the centre of the board, equilibrium
what is the mass of an object
a measure of the matter in the object
Kg
mass remains constant no matter the gravitational field
what is inertia
the resistance to a change in velocity
what does a greater mass mean
the greater its resistance to a change in velocity (it’s inertia)
what is weight
a force
N
experienced by a mass due to a gravitational field
Weight {}
W =mg
approximate value for gravity on the moon
1.6NKg^-1
what are the units for gravitational field strength
NKg^-1
what is a moment
the turning effect of a force around a turning point
force * perpendicular distance from the line of action of the force to the point
moment {}
M = Fd
Nm = N * m
what does the principle of moments state
for a body to be in equilibrium the sum of the clockwise moments must equal the sum of the anticlockwise moments about any point
if the moments on an object aren’t balanced what happens to the object
it explodes!
just kidding.
it will turn
what is a lever
where an effort force acts upon a load force by means of a rigid object rotating about a pivot
why are levers useful
less force is needed for the same amount
since they increase the distance from the pivot at which force is applied
useful in situations where a large turning effect is needed ( spanners, wheelbarrows ect.)
what is a couple
pair of forces of equal size which act parallel to each other in opposite directions
doesn’t cause any resultant force, produces a turning effect
d is the full distance
F is one of the F’s
what is the centre of mass
the single point through which you can consider the whole weight of an object to act
how can you find the centre of mass of an object
hang the object freely from a point
draw a vertical line downwards from the point of suspension, use a plum bob to get the line completely vertical
hang the object from a different point
draw another line
where the lines cross = centre of mass
when would an object topple over
if the line action of its weight falls outside it’s base area
since a resultant moment occurs which provides a turning force
describe a stable object
will have a low centre of mass
will have a wide base area
describe how forces act on supports
force acting on each support won’t be the same
the closer to the centre of mass the support is, the stronger the force acting on the support
principle of moments
what is speed
how fast something is moving regardless of direction
what is displacement
how far an object’s travelled from it’s starting point in a given direction
what is velocity
the rate of change of an object’s displacement
what is acceleration
the rate of change of an object’s velocity
state the 4 main suvat equations
v = u + at
s = ut + 1/2at^2
v^2 = u^2 + 2as
s = (u+v)/2 * t
when are the suvat equations used
when acceleration is constant
what does the gradient of a displacement-time graph show
velocity
if the gradient of a displacement-time graph isn’t constant ( a curve ) what does it mean
the object is accelerating
the velocity is constantly changing
what is an object’s instantaneous velocity
how do you find it on a graph
it’s velocity at a particular moment in time
tangent to the curve at that point
what is the average velocity
total displacement / total time
what is the gradient on a velocity-time graph
acceleration
what does uniform acceleration look like on a graph
straight line
steeper gradient - greater acceleration
what does the area under a velocity-time graph represent
speed-time
displacement of the object
distance travelled by the object
how is non-uniform acceleration shown on a graph
non-constant gradient
curve on velocity-time graph
main advantages of data loggers
data more accurate
human reaction times don’t have to be accounted for
much higher sampling rate (more reading can be taken per second)
data can be processed and displayed in real time
what is freefall
when there’s gravity acting on an object and nothing else
‘motion of an object undergoing acceleration g’
who’s this galileo guy
he conspired that all objects fall at the same rate
free-falling objects fell too quickly for him to be able to take any accurate measurements, air resistance always came in the way too
he measured time it took for a ball to roll down a smooth slope
observed the distance travelled was proportional to the square of the time taken
accelerating at a constant rate
determining g using free fall
set up electromagnet in a circuit with switch
ball bearing
measure height h
release ball bearing and simultaneously begin the timer
when ball bearing hits trapdoor stop the timer
repeat few times to calculate an average
repeat for different heights h
plot a graph of height against s^2
gradient = 1/2 g
what are common sources of error in the determining g required practical
random error - uncertainty of using rulers
small and heavy ball bearing - air resistance can be assumed to be negligible
stopwatch - reaction times can heavily skew results
what is a projectile
an object given initial velocity and then left to move freely under gravity
horizontal and vertical components are independent of one another
why do projectiles follow a curve
horizontal velocity remains constant
vertical velocity affected by acceleration due to gravity , ‘g’
air resistance in projectile motion
causes a drag force that acts in opposite direction to motion and affects trajectory of a projectile
vertical component - drag affects the maximum height the projectile will reach
a block is at rest on the floor
describe the forces acting up on it
weight - force of block on floor due to gravity
support force - force on block due to floor
how can you calculate the weight of a beam/ metre ruler
weight acts through centre of mass
so at 0.5m
then apply principle of moments
upwards force =
downwards force
two support problem
if centre of mass is midway between beam, weight is shared equally between the two supports
if not, ~~↓ = ~~~~~~~~↑
what does it mean to say a body is in stable equilibrium
why?
is the body is suspended and then released it returns to it’s equilibrium position
the line of action of weight of the object feels the need to pass through the point of support
when the object is at rest
the centre of mass of an object is directly below its point of support
what is tilting
when an object at rest on a surface is acted on by a force that raises it up on one side
Fd > weight*(base/2)
clockwise > anticlockwise
what it toppling
if tilted too far an object will topple
line of action of the weight passes beyond the pivot
2 conditions for equilibrium
resultant force must be zero
principle of moments must apply. turning effects balance out
as speed increases how is air resistance affected
air resistance increases
proportional to square of speed
what is meant by terminal velocity
when the forces acting on a falling object become balanced
zero acceleration
falling at a constant velocity
