Forces 2 (Phy) Flashcards
momentum (+ eq.)
the measure of the tendency of an object to keep moving / how hard it is to stop moving
= mass x velocity
p = mv
unit for momentum
kgm/s
conservation of momentum
total momentum before a collision = total momentum after a collision
(unless acted on by a external force)
impact force =
change in momentum over time
= mv - mu / t
if mass stays same:
m(v-u/t)
impact forces: greater mass & same force
increases time
stopping distance
how far a car travels before it stops
= thinking distance - breaking distance
for a car to stop moving …
kinetic energy has to change into heat energy at the brakes and tyres of the car
thinking distance
distance the car travels in a split second between a hazard appearing and driver applying the breaks
what affects thinking distance
how fast car is gong
how intoxicated you are
your concentration
braking distance
distance car travels during deceleration whilst breaks are being applied
what affects breaking distance
speed of car
mass of car
condition of brakes
condition of tyres
condition of road
how does thinking distance change with speed
= 3/10 x speed (mph)
stopping distance at 20
12
stopping distance at 30
23
stopping distance at 40
36
stopping distance at 50
53
stopping distance at 60
73
stopping distance at 70
96
fluid
a substance that can flow (eg liquid or gas)
3 stages of falling
at the start, the object accelerates downwards due to the force of gravity
as the object’s speed increases, frictional forces such as air resistance or drag increase
at terminal velocity, the weight of the object due to gravity is balanced by the frictional forces, and the resultant force is zero
The weight of an object does not change as it falls, as long as it stays whole.
object with ____ mass fall faster through a fluid
greater- more weight and same fluid resistance
how would i improve my experiment about terminal velocity?
more repeats
do it over a larger distance
elastic limit
past it:
- materials no longer show elastic behaviour (do not return to original size when stretching force is removed)
- materials permanently deformed
- material weaker (fracture of some atomic bonds)
Hooke’s Law
f = kx
force = spring constant x extension
spring constant (k)
stiffness of spring
- units of Nm-1
- higher k is, the stiffer the spring
- materials with higher k need a large force for a given extension
how to interpret force extension graph
force on y axis, extension on bottom !!
gradient is k
therefore:
steeper line means stiffer (high k)
gentle line means less stiff (low k)
why is hookes law important
- for construction of structures (eg bridges buildings)
- for vehicle design (deformation of materials)
lever
long bar that pivots
- when you push down on one side (effort), the load (object on the other end) moves up
moment (+ units + eq.)
turning force
- Nm
= force x perpendicular distance
how to do balanced moments question
F1 / F2 = L2 / L1
size and gears
If the driven gear is made larger is will rotate more slowly but with a greater moment. For example, a low gear ratio on a bike or car.
If the driven gear is made smaller it will rotate more quickly but with a smaller moment. For example, a high gear ratio on a bike or car.
gears
Gears are wheels with toothed edges that rotate on an axle or shaft. The teeth of one gear fit into the teeth of another gear. This lets one gear turn the other, meaning one axle or shaft can be used to turn another shaft.
The forces acting on the teeth are identical for both gears, but their moments are different
centre of mass
the point where the weight of the object acts
centre of mass of different shapes
even sided: join up opposite corners
odd sided: join up point to midpoint
explosion
when the momentum before the collision is 0, the momentum after will be zero
velocities will be equal and in opposite directions
car safety
helmets increase impact time
seat belt increases time to stop - reduces force
seatbelt is on chest so spreads force out, otherwise force would be applied to head
airbag does both these
why are soft surfaces good for collisions
cushioning effect- duration of impact is longer
