Flashcards in Topic 4 - Momentum, Energy, Work and Power Deck (31):
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?
Is momentum a vector?
What is the equation for momentum?
Momentum = mass x velocity
What is momentum measured in?
What is the relationship between momentum before and momentum when no external forces act?
-Momentum is conserved
What is the equation relating force, chance in momentum and time?
Force = change in momentum / time
What is force measured in?
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?
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
What is all energy measured in?
What is the equation for gravitational potential energy?
GPE = mass x g (gravitational field strength, 10 N/kg) x height
What is the gravitational field strength on earth?
What is the principle of conservation of energy?
Energy can never be created nor destroyed, only transferred from one form to another
What is a principle of energy being useful?
Energy is only useful when it's transferred from one form to another.
How does energy transfer in a falling object?
-GPE converted to KE (principle of conservation of energy)
-Some GPE likely dissipated as heat from air resistance
-Kinetic energy gained = Gravitation potential energy lost