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Force
& Motion (Safety first (Safety helmets
For cyclists, horse…
Force
& Motion
Acceleration
Newton's 2nd Law
Acceleration, proportional to resultant force
Acceleration, inverse proportion to object mass
Therefore F = ma (1N, gives 1kg, 1m/s^2 of acceleration)
Inertia
Tendency of object to rest or be in constant velocity
Inertial mass - difficulty of changing object's velocity
Where m = F/a
Accelerate/Decelerate
Resultant force in same direction of velocity = accelerate
R. force in opposite direction of velocity = deceleration
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Forces &
Braking
Road forces
Use accelerator pedal to vary engine force
Braking force to stop car depends initial speed, mass
Incr. speed - Incr. deceleration needed
Incr. mass - Incr. force for same deceleration
Stopping Distance
Thinking distance + Braking distance:
- Time for driver to react (speed x react time)
Therefore T.distance is prop. to speed
- Distance car travels during action of braking forces
Factors
- Tiredness, alcohol, drugs, mobile phones incr.
Thinking distance (react time)
- Incr. initial speed - incr. Braking distance (s = -u^2/2a)
- Wet/icy, poor weather, drivers break less (avoid skidding)
- Worn brakes, tyres - parts are less effective, incr. B distance
Vehicles and Forces
To prevent skidding - decelerate <6 m/s^2
(u^2 +2as = v^2)
Vehicle mass = 1-38 tonnes, therefore
B force = 6-250 kN; same range to accelerate
Momentum
Formula
In general, an object with high momentum is difficult to stop
Because p = mv
(Momentum = mass x velocity)
Investigate collisions - conservation of momentum
A is given a push to collide and stick w/ B
Measure A before, after collision
If same mass - A's velocity is halved after collision
If B has double mass - A's velocity is divided by 3
Therefore momentum after collision = before collision
This only occurs in a closed system
-
Investigate 2 - vector
A & B push off each other
Both same mass = both same speed, distance
B has double mass = travels half A's distance
Therefore mv is the same for both trolleys
Yet they move in opposite directions
Therefore momentum has magnitude and direction
Conservation
of Momentum
More Collisions
Bowling ball hits skittle, skittle gains speed, ball loses
Equal/opp force exertion w/o external forces
CoM only applies w/ external forces
Applications - explosions
p(A) = mass x velocity
p(B) = mass x velocity
p(before) = 0
p(A) + p(B) = 0
Therefore A mass x A velocity = -(B mass x B velocity)
minus for opposite direction with same momentum
In action
Shell fired from artillery, barrel recoils back
Recoil slowed by spring, lessens backward motion
Impact forces
Crumple zone
Front and rear of car, lessens impact force
Front impact - car's momentum is reduced
Rear impact - car's momentum is increased
C zone increase impact time, lessens force
Impact time
In collision:
Impact force acts for given time
Soft pads incr. impact time as a cushion
Momentum reduction over longer time
Change per second is less, I.force = change per s
Longer impact time = reduced I.force
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In Collision
Objects exert equal/opp forces, total p is the same
Changes of p of both objects are the same w/o ext. forces
Safety first
Safety helmets
For cyclists, horse riders etc. for falling
Incr. impact time w/ head falling up to 10 m/s
Reduce impact force
Car Safety
W/o seatbelt can lead to committing head thro windscreen
Impact time incr. w/ seat-belt instead of "short sharp" impact
Seat-belt across chest = force spread on chest, instead of head
Air bag incr. SA of impact on upper body & incr. impact time
C zone, impact bars, collapsible steering wheel incr. time
Child Seats
For children <12 y/o or <1.35 m, different chairs for:<9 months, <4 y/o, >4 y/o; Babies must face back<4 y/o should be in child seat fitted to back
To reduce death, serious injuries of children in cars
Children before were more likely to be seriously injured
Most common during school run
Road Safety
Harsh braking = skidding; Td prop to u, Bd prop. to u^2
Stopping distance is min. safest distance
Playground Safety
Cushioned surfaces under swings, slides
Incr. impact time compared to concrete, decr. I force
Gym crash mats have same effect as a cushion
Forces &
Elasticity
Concept
Object shape can be changed
Through stretching, bending, twisting, compressing
Some objects are elastic - return to original shape
These objects are "elastically deformed" (rubber etc.)
Polythene bag doesn't - "in-elastically deformed"
Only elastic materials can return to there original shape
after being deformed
Extension
Difference of object length from original length
Normally plotted on x-axis w/ weight on y-axis
If line is straight, goes through origin = d.p.
Where double weight, doubles extension
Hooke's law
Spring extension is d.p. to force applied under
limit of proportionality
(the point of which the string stretches more than expected)
Rubber & polythene's limit is low becoming non-linear earlier
F = ke
where k = spring constant (force per unit extension)
Stiffer spring = higher prop. (ofc under limit)