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level: Everyday Speeds and Accelerations

Questions and Answers List

level questions: Everyday Speeds and Accelerations

QuestionAnswer
How can you: -Convert from Miles per Hour [mph] to Kilometres per Hour [km/hr] -Convert from Kilometres per Hour to Meters per Second [m/s]-Simply Multiply by 1.6, as 1 Mile = 1.6 km. [30 mph = 30 mph x 1.6 = 48 km/h] -Simply DIVIDE by 3600, from Hours to Seconds - and then MULTIPLY by 1000, from Kilometres to Meters. [OR Divide by 3.6] [48km/h / 3.6 = 13 m/s]
What are the Predicted Speeds for 1. Walking 2. Running 3. Cycling 4. Cars in a Built up Area 5, Cars on a Motorway 6. Trains 7. Breeze [Wind Terminology] 8. Gale 9. Sound in Air1. 1.4 m/s OR 5km/hr 2. 3 m/s OR 11km/hr 3. 5.5 m/s OR 20 km/hr 4. 13 m/s OR 47 km/hr 5. 31 m/s OR 112 km/hr 6. 55 m/s OR 200 km/hr 7. 5 m/s 8. 20 m/s 9. 340 m/s
EXAMPLE: A Car from rest, Accelerates to 50 km/hr as Fast as Possible -Predict its Acceleration in m/s- 50 km/hr = 50 km /hr / 3.6 = 13.88... m/s -You have to Guess the Time it took for it to reach 13.88 m/s, so pick a Number that is Sensible - 2 Seconds in this Case -So we use Acceleration = Change in Velocity / Time Taken -Acceleration = 13.89 m/s / 2 = 7 m/s² [1 Sig Figure]
Why can Big Decelerations be Bad?-A Large Deceleration can be Lethal due to it needing a Large Force, because Force = Mass x Acceleration. -Also, the Brakes may Overheat as a Result of it, as Lots of Energy is Quickly Transferred to the Thermal Energy Store of the Brakes -Vehicle may also Skid, which isn't exactly Fun.
So what can be done to Prevent a Large Deceleration?-If you can make the Object Slow down - eg. Slow the Car down over a Long Time, then the Deceleration will be Lower as because Acceleration = Change in Velocity / Time. -If the Momentum Changes over a Longer Period, then the Force will be much Smaller as because: -Force = Change in Momentum / Time
Why are their Safety Features in Cars and more?-It is to Increase the Collision Time, which again Reduces the Deceleration and therefore the Force, which Reduces the Chance for Injury -Seat Belts will Stretch, Air Bags will slow you Down and Crumple Zones will well Crumple up Easily, making the Time Taken to Increase [Lowers Force and Acceleration] -Bike Helmets also has a Crushable Layer of Foam, which again Increase the Amount Time Taken for the Head to Stop. -Shin Pads also Deform when Hit, reducing the Deceleration and Force on your Leg.