Topic 4 - Momentum, Energy, Work and Power Flashcards
What is stopping distance?
The distance it takes to stop a car, made up from thinking distance + braking distance
What is thinking distance?
The distance the car travels in the time between the driver noticing the hazard + applying the brakes
What factors affect thinking distance?
- Reaction time: affected by tiredness, drugs, alcohol, age
- Fast you’re going: faster you go, further thinking distance
What is braking distance?
The distance the car travels during its deceleration whilst the brakes are being applied, time between the brakes applied + car stopping
What factors affect braking distance?
- Speed: faster = further braking distance
- Mass of vehicle: w/ same brakes, heavy = longer to stop
- Quality of brakes
- Grip: friction between tyres + road surface to stop (depends on road surface, weather, tyres)
What experiment could you do to investigate how much frictional force different surfaces provide?
- Add masses 1 by 1 to mass holder to provide a force on block until eventually slides
- Amount of force (amount of mass) needed to slide block depends on amount of friction between block + surface (like tyres on road surfaces)
- Can use different surfaces
- Smaller frictional force between block + surface = smaller force to make block slide
How does a greater mass/greater velocity affect momentum?
More momentum of an object
What is a vector?
- Has size
- Has direction
Is momentum a vector?
Yes
What is the equation for momentum?
Momentum = mass x velocity
What is momentum measured in?
kg m/s
What is the relationship between momentum before and momentum when no external forces act?
- Equal
- Momentum is conserved
What is the equation relating force, chance in momentum and time?
Force = change in momentum / time
mv-mu F= t
What is force measured in?
Newtons
Why are protective features put into cars?
- Slow people over a longer time when they crash
- Take a longer time to change momentum, decreasing force
What protective features are out into cars to increase stopping distance and how does this make the car safer?
- Crumple zones: crumple on impact, increase time taken for car to stop
- Seat belt: stretch so increase time taken for wearer to stop reducing force on chest
- Air bags: slow you down more gradually
How can bubble wrap and an egg be used to demonstrate how crumple zones work?
- Dropping egg on hard floor means they shatter
- Drop onto cushions in foam or build crumple zone of cardboard + bubble wrap
- Collisions happen over longer time so force on egg is smaller
- Reduces the force on the egg
What causes work to be done?
When a force moves an object, energy is transferred and work is done.
What is work done measured in?
Joules, J
What is the formula for work done?
Work done = force x distance moved in the direction of the force
What is the formula linking power, work done and time taken?
Power = work done / time taken
What units can be used to measure power?
Watts, W or J/s (joules of energy per second, 1 W = 1 joule of energy per second)
What is the equation for kinetic energy?
Kinetic energy = 0.5 x mass x velocity^2
Initial kinetic energy = work done by brakes to stop vehicle. What is this in symbols and what happens to work done if you double your speed?
- 0.5 x m x v^2 (velocity) = F (maximum braking force) x d (braking distance)
- Double speed = 2xv, and as v^2 KE increases by factor 4
- F is always maximum possible braking force so d must be increased by factor 4 to balance
- So double speed = 4 x braking distance