mechanics Flashcards
newtons third law pairs
-same size
-same line of action but in opposite directions
-same time of action
- same type of force
-acts on diff bodies
FAB = -FBA
2 relationships between forces for object in equilibrium
- resultant force 0N
- resultant Torque 0 (ACW m =CWm)
in moments - talking about why a force / distance must be greater / lesser to balance
- subject of questions relative distance (e.g. W is a greater distance)
- subjects __ must be > or < for moment to be equal / resultant moment 0N (e.g. W force must be less)
if an object moves along the system in equilibrium (e.g. girl walks along diveboard) - how to describe moment
-moment of object’s weight increases
moment definition
- force x perpendicular distance
- from line of action of force to pivot
principle of moments (3)
- for body in equilibrium
- SUM of ACW moments = SUM of CW moments about a POINT/PIVOT
comparing forces
- distance pivot (1)
- gives large enough / small enough moment
two conditions for object in equilibrium
- resultant force = 0N
- resultant Torque =0Nm ABOUT ANY POINT
consequence if object is not in equilibirum
- accelerate
- rotate w/ angular direction
direction of tension
–> force case of hanging mass
- the tension will point away from the mass in the direction of the string/rope.
- In case of the hanging mass, the string pulls it upwards, so the string/rope exerts an upper force on the mass and the tension will be in the upper side.
always add what on angles in vector diagrams?
-directions NSEW
what will happen to forward force to maintain constant acceleration?
- forward force increase
- air resistance increases w/ speed
- driving forward force > drag force ( for constant acceleration)
- otherwise, the net resultant force would decrease
what is constant acceleration?
velocity changes at a constant rate (net force must stay the same)
when doing distance under graph
-always do the WHOLE under graph even if it looks as if it starts mid y axis
when describing motion always
-include figures even if it looks as though you shouldn’t;
when cars start at different points on graph (3) to explain why one reaches a maximum distance ahead of other
- car (B) initially slower
- therefore distance apart increases
- cars reach same speed after (time)
how to collect data for sprinter velocity time graph
- record time to reach set distance
- d/t graph measure gradient
on elevated plane vector questions the angle is in between
-given W and vertical component
gravitational potential energy (or any energy lost) points (3)
- lost as heat energy to the surroundings
- frictional forces / air resistance / drag between (2 surfaces)
- WORK DONE against or by resistive force
if there’s a delay in distance time graph e.g. sprinter w/ pistol reason is
reaction time
why does changing mass not affect g (2)
- weight and mass directly proportional w=mg
- -> double weight double mass therefore factor of 2 cancels and g remains the same
when talking about difference between distance and displacement in graph
say d is scalar and s is vector the graph
when describing motion
-always comment on direction (e.g. opposite direction) to other half of graph
when talking about why an object might not travel as far in one direction (e.g. horizontally)
talk about air resistance causing deceleration in HORIZONTAL
for s = ut +1/2 at^2 when t is unknown
-create a quadratic + solve for unknown t
for object thrown from cliff initial velocity assumed as
0 ms
if not given acceleration and says constant acceleration assume acceleration to be
0ms-2
- A ball is thrown vertically upwards at 25 m s−1. Find the length of time for which the ball is above 3 m from the point of projection.
(why two solutions for t ?)
There are two solutions because there are two times when the ball is at a height of 3 m - once on the way up and once
on the way down. The difference between these times is the time spent above 3 m
Relationship displacement y and time x in free fall
s = ut +at^2 /2
S=(0)t + at^2
S=t^2
Y is proportional to square of x
3 marks
–> explaining why for smaller horizontal velocity == smaller distance travelled
(vertical motion independent horizontal)
(1) same acceleration downwards due to gravity = same vertical speed ( as same mass)
(1) in air for same time
(1) s=ut where u is horizontal speed
energy changes in cannonball being fired
- GPE- Ke
- to heat / thermal
why if velocity diver independent mass if air resistance insignificant
- all gpe transfer to KE
- no transferred to heat
- mass cancels
if the object in question is not stated to be at rest final velocity must not be
0
–> measuring velocity BEFORE impact
six marker - ball bearing in oil
-ball decelerates down in the fluid - force due to friction acts upwards + > downwards force on weight of ball
-resistive force = upwards + decreases
{]deceleration decreases to 0:
-drag force decreases as speed decreases
-until equal
-grad graph = acceleration + ball moves at terminal velocity
why acceleration decreases as move downslope(2)
- component weight downwards = acceleration
- move further down component decreases
- air resistance increases as speed increases
Galileo’s experiment - frictionless track why ball returns to same height (3)
ALL -GPE –> KE –> GPE (1)
- -> no work done against friction / resistive forces (1)
- -> as initial GPE= final GPE + mg is constant h same (1)
what to say when air resistance is negligible in terms of energy
no !WORK DONE ! against resistive forces
Key words for Newton first law of motion
- no external unbalanced resultant force acting on it
- constant velocity in a straight line
gravitational field strength
vector
force per unit mass
remember when horizontal component of vertical force increases the vertical force increases in magnitude at
an angle (infleunced by angle e.g. V sinx)
moments in relation to what is the centre of mass
point at which resultant torque / moment is 0
3 marker on moments + toppling
- feature
- what it does (eg. sufficient ACW moment)
- prevents what ( toppling !CW!)
FOR conservation moments
momentS (!need S!)
in moments for example girls weight consider as
moment of girls weight
The figure below shows a stationary metal block hanging from the middle of a stretched wire
which is suspended from a horizontal beam. The tension in each half of the wire is 15 N.
–> horizontal component =
0N – as horizontal component equal magnitudes but diff directions cancel out
(vertical is sum of vertical components)
be wary of difference between
constant acceleration and constant speed
elastic collision molecules gas change in momentum
2mv
Use Newton’s laws of motion to explain how the molecules of a gas exert a force on the
wall of a container.
You may be awarded additional marks to those shown in brackets for the quality of written
communication in your answer.
force is exerted on molecule by wall (1)
to change its momentum (1)
molecule must exert an equal but opposite force on wall (1)
in accordance with Newton’s second or third law (1)
don’t refer to as initial velocity in momentum questions always refer to as
momentum (e.g. initial monetum greater)
when saying impulse is same also say
change in momentum is the same
when saying ke lost also say
change in ke
explain with ref to momentum why test would specifiy whihc surface plinth is on
Impact time can be increased if the plinth material is not stiff✓
Increased impact time would reduce the force of the impact. ✓
why acceleration decreases as skateboarder moves a to b
- component weight down = acceleration
- component decreases as skateboarder moves further down
- air resistance / friction increases w/ spee
why in fluid ball decelerates
- friction force acts upwards
- friction force due to fluid > weight ball
- resistive forc decreases
why deceleration decreases to 0 in fluid ball
-balls speed decreases friction force decreases-until equal force - balls at constant terminal v
what is gravitational field strength ?
force per unit mass ✓
a vector quantity ✓
ALWAYS REMEMBER
normal reaction force on pivot upwards
newtons first law 2 terms
0 resultant force
0 acceleration
always remmeber on draw on force
include weight + normal reaction
F = ma when mass is
Constant
suggest w/ reason how resultant force changes as speed increases
- decreases
- air resistance directly proportional to speed
use newtons laws of motion to explain why the vehicle has max speed (3)
- two forces are equal (in magnitude) (1)
-the resultant force is zero (1)
hence constant/terminal velocity (zero acceleration)
in accordance with Newton’s first law (1)
effect on forwarding + sideways speed of the object when acted on by force at right angles to the motion
- forward speed unchanged
- sideways speed increases
3 ways newtons third law pairs are same
they have the same line of action
they have the same magnitude (not size)
the forces are of the same kind
four thrusters on a rocket fired simultaneously in same direction - explain how propellant produces thrust (3)
==> Chemical combustion of propellant/fuel or gases produced at high pressure
- Gas is expelled / expands through nozzle
- Change in momentum of gases escaping
- equal and opposite to change in momentum of the spacecraft
- Thrust = rate of change of change in momentum
Q13.
An object falls freely from rest. After falling a distance d its velocity is v. What is its velocity after it has fallen a distance 2d?
root 2 v
-equate
explaining newtosn first law from graph
(first law) ball travels in a straight line at a constant speed / constant velocity
/ (maintains) uniform / no change in motion / zero acceleration
there is no (external) unbalanced / resultant force acting on it
always draw reaction force up at
pivot
velocity-time graph used to find MAXIMUM acceleration
steepest gradient
at start of projectiles questions always state
vertical + horizontal velocities seperate
why air bag is safer (3)
moving driver has momentum (1)
in sudden impact momentum must be lost in v. short time (1)
F = Δ(mv) / Δt (or F = ma) (1)
air bag increases stopping contact time (1)
hence reduces force (or reduces force by decreasing the deceleration) (1)
on show that questions
(to 4 sf minimum-no up)
to describe how pivot placed at a position that means moments are balanced
- the center of mass of beam + box is the pivot
- CW=ACW Moments
for saying range will be greater when projectile given vertical w/o horizontal (4)
component of velocity upwards ✔
rock will spend longer in the air ✔
greater t ✔
therefore the range is greater ✔
speed of air same why does momentum change (2)
- momentum vector
- direction changes
why would decelerating force on air by deflector planes (that change momentum of air) not be constant (2)
- vol air per second decreases
- smaller rate of change of momentum F = change in momentum / t
Describe and explain what happens when a moving particle collides elastically with a stationary particle of equal mass.
- the incident particle stops (loses all its momentum or KE)
- the stationary particle moves off with the same velocity (speed) as the incident particle (gains same momentum or KE as incident particle had)
- original KE = ½mv2 = final KE
Collisions between neutrons and uranium nuclei can also be elastic. State, and explain briefly, how the speed of the uranium nucleus after impact would be different in the case of an elastic collision.
(uranium stationary initially + greater mass)
- neutron will rebound / have velocity / momentum to the left
- momentum transferred to the uranium will be greater
- velocity of uranium will be greater
projectile has max pe at
the top of its path
higher Young modulus =
stiffer material
what are couples NOT
perpendicular
RF of a couple
0
d for moment of couple
distance between lines of actions of forces
object fall over if
centre of mass not above its base
what’s an explosion
collision in reverse
what processes are explosions
–> what does ke do and where does ke of fragments come from
inelastic
increases
form internal energy of system
eqn for no acceleration
speed distance time
how to show directly proportional method 1
Read-off points (of line of best fit for) x1 and x2 compare with corresponding y1 and y2 , compares the ratio of the x terms to the ratio of the y terms; if equal then directly proportional
how to show directly proportional method 2
Determine the constant of proportionality for at least two points (on line of best fit) and compare, where constant exists then directly proportional ✔
how does data show distance greater refer to what
!EQUATION!
- how is time greater
- how is speed greater
- speed and time directly proportional to distance
always give
decimals not fractions !
remember
Can’t use suvat in km/h only m/s
in newtons second law - why would gas exert force is displaced by parachute
- gas momentum is changing - accelerated - change in direction /vecloity
- therefore F=MA so force SO required to change momentum
in newtons second law resultant force on object equal to rate of change in momentum IF
change in momentum in same direction as resultant force therefore acceleration in direction (ALWAYS SAY SAME DIRECTION)
larger angle of projectile =
smaller horizontal v
bigger vertical v
DUE TO COSINE AND SINE GRAPHS 0-90
2 marker on explaining why you drew projectile
- angle in relation to velocity
- where it ends up
energy changes when block slowing down
ke to heat
measuring speed on Galileo track
equate ke = mgh
cancel m
v^2= gh
efficiency
useful output / total input - don’t minus
2 marker on how graph of velocity time shows acceleration constant
GARDIENT = ACCELERATION
constant gradient
for increasing acceleration force has to be
LARGER can’t just say increase this increase that
