P8.1 Flashcards
How would you measure the speed or a 100m sprinter?
- method = electronic time
- distance = tape measure
- time = pressure sensor to st art and laser broken to end
How would you measure the speed or a car on a road?
- method = speed camera/speed gun
- distance = trundle wheel to make road or d = s x time taken for reflection/ 2
- time = time between 2 photos or pulses
How would you measure the speed of a cyclist?
- method = wheel sensor
- distance = diameter of wheel
- time = magnetic sensor to detect rotation
How would you measure the speed of any moving object?
- method = Satnav
- distance = comparison of 3 satellites to distance
- time = electronic timer
How do you convert from mph to km/hr?
- convert 30mph to km/hr
- 1.6km per mile
- x by 1.6
- 48km/hr
How do you convert from km/hr to m/s?
- convert 48km/hr into m/s
- divide by 3600 to turn hour into seconds and then x 1000 to change from km to m
- so just divide by 3.6
- 13.3 ≈ 13m/s
How do you calculate acceleration?
Change in velocity / time
What is a typical speed for walking?
- 1.5 m/s (5.4km/hr or 3.4mph)
What is a typical speed for running?
- 5 m/s (18km/hr or 11mph)
What is a typical speed for cycling?
- 7 m/s (25km/hr or 15mph)
What is a typical speed for cars in a build-up area and a motorway?
- build up areas = 13m/s (47km/hr or 30mph)
- motorway = 31m/s (112km/hr or 70mph)
- average = 22m/s (79km/hr or 50mph)
What is a typical speed for a train?
- up to 55m/s (88km/ hr or 92mph)
What is a typical speed for a breeze?
- 5m/s (18km/hr or 11mph)
What is a typical speed for a gale?
- 20m/s (72km/ hr or 45mph)
What is a typical speed for the speed of sound?
- in air = 340m/s (1224km/hr or 765mph)
- 330 - 340m/s
What’s the difference between the terminology of precise and accurate?
- precise = small range when repeated
- accurate = close to the true value
What is the reaction time?
- time taken from seeing something to the reaction (brake/pressing stopwatch button)
- human reaction time = 0.2 secs
How can we measure human reaction time?
- hold ruler between thumb and forefinger and forefinger should line up with 0 (3rd person at eye level may check if it’s lined up)
- suddenly drop ruler and close thumb and finger when you catch it
- measurement on ruler at point it was caught is how far ruler dropped in time it took you to react = longer distance = longer reaction time
- use equation v2 - u2 = 2ad where u is 0, a is 10m/s and d is where you caught it and use a = change in v / time and rearrange to time = change in v / acc where ‘answer to earlier calculation’ is v and Acc = 10m/s
- do lots of repeats and find mean distance, fair test (people, ruler)
What is the thinking distance?
- distance traveled in the time it takes from seeing a potential problem and to start to apply the brakes
- can be effected by the reaction time and the speed
- speed increases = thinking distance increases at same rate as the drivers reaction time will stay constant but if higher speed = further you go in that time (can be effected my tiredness) as d=st
What can affect thinking distance?
- drinking alcohol/using drugs
- being tired
- distractions such as eating/drinking
- using a satnav/ radio can mean increased speed
(More distracted/ speed = distance the car travels is longer)
Estimate the thinking distance for a car travelling at 50mph
- 50mph in m/s is 22.2 m/s (x1.6 and /3.6)
- thinking distance = speed x reaction time (0.2 seconds) = 4.5m (2s.f)
Calculate the reaction time for a student who gets 25cm using the ruler drop
- square root of 2 x 10 x 0.25 (convert to m) = 2.23 m/s
- speed = d/t so 0.25/2.23m/s = 0.1118s or 0.11 s to 2 s.f.
What is the braking distance?
- distance taken to stop once the brakes have been applied
- braking distance and speed = squared relationship = if speed doubles then braking distance increases 4- fold (2 squared) and trebles then increases x3 (a little less)
What can affect the braking distance?
- speed = faster you’re going, the further it takes to stop
- mass = more mass so stops slower
- condition of the brakes = worn/faulty can’t brake with as much force
- grip of the tires = more likely so skid when road is dirty, ice or wet or if they’re bald (tires = min. tread depth of 1.6mm)
- ice or leaves on road could reduce grip
What is the stopping distance?
- total distance travelled from moment deliver sees a problem to coming to a stop
- thinking + stopping distance
What affects the stopping distance?
- drivers need to leave space in front so they can stop safely which is at least equal to stopping distance to their speed
- speed limits = important = increases then thinking distance increases
- braking distance and speed = squared relationship
How do brakes stop a car?
- the brakes of a car do work on the cars wheels
- this transfers energy from cars ke store to thermal energy store of the brakes
- stop a car = brakes must transfer all of this energy
- doubling the mass doubles braking distance
How can we calculate the braking force or braking distance?
Energy in cars ke store = work done by the brakes
- 0.5 x m (mass) x velocity squared = f (braking force)x d (braking distance)
What’s the relationship between the thinking and braking distance?
- energy 10mph increase in thinking distance, the braking distance increase by 3m
- linear relationship
How much does a car, single decker bus and loaded lorry normally weigh?
- car = 1000kg
- single decker bus = 10,000 kg
- loaded lorry = 30,000 kg
How does the deceleration change when the speed with a high speed compared to a driver with a low speed?
- the lower speed driver = smaller thinking and braking distance and same reaction time
- higher speed driver = higher thinking and driving distance and same reaction time
- the gradient p/ deceleration is the same as the maximum force applied on the brakes hasn’t changed
How can we interpret velocity time graphs for braking/thinking distances when an object decelerates?
- stationary velocity = brakes applies as reaction time = area underneath gives thinking distance
- deceleration = area underneath gives braking distance
- gradient is same as force applied by brakes is same
What is a collision?
- a large negative acceleration or large deceleration
How do seal belts allow you to come to a slower stop?
- when a car stops you continue to move forward and the seatbelt stretched to bring you to a slower stop
- if you don’t wear one then you accelerate at the original speed of the car
- need to be replaced after a crash
What happens if the deceleration is large?
- compression injuries can occur from the seatbelt or internal organs can be damaged as they collide with the ribs
- as a large deceleration needs a large force as f=ma
How can a force on an object be lowered?
- lowered by slowing the object down over a longer time such as decreasing deceleration as a= change in speed /time and f =ma
- the longer it takes for a change in momentum, the smaller the force acting
What affects the force you experience from a collision?
- faster you go = greater force experienced in crash and more injuries
- depends on the time taken for the collision to come to a stop
How can we reduce the force using features?
- crumple zones (diff density material so squish and longer stop and less force on people)
- air bags = inflate so head hits something that allows you to deflate it slightly = slower stop and less forces acting on you
How do bike helmets and shin pads reduce injury?
- helmets = crushable layer of foam = increases time taken for your head to stop = reducing deceleration and forces acting
- shin pads = pad deforms of something hits it= reduce deceleration and force on leg
What is the g-force?
- acc that you experience is about twice the acc due to gravity (9.81m/s^2) so you feel a force equal to times ‘X’ of your weight
- 2g = 2 times weight
- 3g = force is 3 times weight
What do safety features in cars do?
- increase collision times which reduce deceleration and forces = reduce injury
- seat belts stretch slightly and air bags slow you down gradually
- some also work by stopping you hitting heard surfaces like the dashboard or the road
What are crumple zones?
- areas at the front and back of a car which crumple up easily on a collision, increasing the time taken to stop
What is the typical reaction time for a person?
- 0.5 seconds or 500 milliseconds
- 1 s = 1000 milliseconds
- so 0.1 second = 100 milliseconds
What happens to the thinking and braking distance when a car doubles in speed?
- thinking distance = doubles
- braking distance = quadruples as it’s proportional to speed squared today
What happens to your reaction time when it’s night and you’re tired?
- increases
Why do we repeat an experiment 3 times and take the average?
- reduces the effect of random error
How can you calculate deceleration knowing the force, speed, reaction time, distance and mass?
- force is mass x acceleration
