P2 Forces (dynamics) Flashcards
define distance and displacement
distance: scalar - how far an object has travelled (eg. 5km)
displacement: vector - how far an object has travelled in a particular direction (eg. 5km east)
typical values of speed for human walking/running/cycling
walk: 1.5 m/s
run: 3 m/s
cycle: 6 m/s
factors affecting a person’s speed (5)
age, health, mode of transport, terrain, length of journey
describe a distance-time graph showing constant speed
straight line
* horizontal = stationary
* diagonal = away/towards the start
describe a distance-time graph showing acceleration
curved line
steeper as speed increases
state equation for average acceleration
acceleration = change in velocity / time
a = ∆v / t
(m/s²) (m/s) (s)
-> change in velocity = final - initial
describe the speed+velocity of an object moving in a circular path
- moves at a constant speed
- there is a change in velocity due to the change in direction
(as velocity is the speed of something in a given direction)
describe a velocity-time graph showing uniform acceleration
straight, diagonal line
describe a velocity-time graph showing constant velocity (steady speed)
straight, diagonal line
moves upwards
describe a velocity-time graph showing increasing acceleration
curved line
moves upwards
gets increasingly steeper
describe a velocity-time graph showing decreasing acceleration
curved line
moves upwards
gets increasingly shallower
describe what the area under a velocity-time graph shows
distance travelled
state the equation for uniform acceleration (used when time is unknown)
final velocity² - initial velocity² = 2 x acceleration x displacement
v² - u² = 2as
(m/s) (m/s) (m/s²) (m)
describe Newton’s 1st law of motion
- an object will remain stationary / at constant velocity when the resultant force is 0N
- acceleration is 0 and velocity is constant
describe Newton’s 2nd law of motion +equation
- the accleration of an object depends on its mass and the resultant force
- resultant force = mass x acceleration
F = m x a
(N) (kg) (m/s²) - resultant force is directly proportional to acceleration (when mass is constant)
define inertia
an object’s tendency to remain stationary/uniform motion - to resist motion
define inertial mass
- the mass calculated from m = F / a (Newton’s 2nd law)
- the ratio of force to acceleration
- how difficult it is to change the velocity of an object (by applying resultant force)
method for investigating relationship between resultant force + acceleration for an object of constant mass (Newton’s 2nd law)
- set up apparatus: dynamics trolley connected to hanging masses by string+ over a bench pulley, metre ruler along bench
- measure a distance (78cm) that the trolley will accelerate over
- apply a force of 1N to the string (hang a mass of 100g)
- release trolley and time over the distance
- repeat twice and calculate a mean time
- repeat for forces of 2 - 5 N
describe Newton’s 3rd law of motion
- if object A exerts a force on object B, B will exert the same amount of force on A to maintain equilibrium
- force exerted by B will be equal in size / opposite in direction / of the same type
define reaction time
time taken between stimulus (seeing the hazard) and the reaction (muscle movement)
range of human reaction time
0.2 - 0.9 secs
define thinking distance
distance travelled by vehicle from stimulus (when a hazard is seen) to when brakes are applied
(reaction time)
factors increasing thinking distance/ reaction time (5)
- alcohol
- drugs (prescribed or legal)
- tiredness
- distractions - phone, passengers, radio, eating
- increased speed of vehicle - thinking distance is directly proportional to speed
define braking distance
distance travelled by vehicle from when brakes are applied to a stop
factors increasing braking distance (4)
- worn-down tyres
- worn-down/faulty breaks
- road conditions: ice, wet, spillages
- increased speed of vehicle
equation for braking distance
force x distance = 1/2 x mass x velocity²
[work done = kinetic energy]
F x d = 1/2 x m v²
(N) (m) - (kg) (m/s)
what happens to braking distance when speed doubles
distance increases 4x
(speed increases x2 so distance increases x2²)
F x d = 1/2 x m x v²
name the main factor affecting stopping distance
speed of the vehicle
equation for calculating stopping distance
thinking distance + braking distance
describe why a vehicle with a greater speed is more dangerous for drivers
use: braking force, deceleration
- the greater the speed of a vehicle, the greater the braking force needed to stop it
- the greater the braking force, the greater the deceleration of the vehicle
- large decelerations may cause brakes to overheat / loss of control (skidding)
describe how brakes heat up when a driver applies force to the brakes
- work done by friction between brakes+wheels reduces the wheel’s kinetic energy
- energy is transferred from wheel’s kinetic energy store to the brake’s thermal energy store
- so brake’s temperature increases
method of calculating human reaction time RP
- partner holds a ruler above you
- position your hand infront of you so the ruler will sit between your index finger + thumb - top of your finger should be level with 0cm on ruler
- partner drops ruler unexpectedly, catch asap
- measure the point at which you caught the ruler above your thumb
- repeat three times and find a mean reaction time
typical results for reaction time for ruler drop test RP
<7.5cm - excellent
<16cm - good
<20cm - average
<28cm - fair
+28cm - poor
state equation for momentum
momentum = mass x velocity
p = m x v
(kg m/s) (kg) (m/s)
describe conservation of momentum +equation
total momentum before an event (collision/explosion) is equal to total momentum after an event - in a closed system
(m1 x v1) + (m2 x v2) = (m3 x v3) + (m4 x v4)
