Power, momentum, and cars Flashcards
What is power?
The rate of doing work
What is a powerful machine?
One that transfers a lot of energy in a short space of time
What is the formula for power?
Symbol equation…
Power= Work done (energy transferred) / (over) Time taken
P=E/t
What is power measured in? What does that equal per second?
Watts = 1 joule of energy transferred per second
How to measure the power output of a person:
A timed run upstairs where you have the weight of the person, the time taken, and the height of the stairs.
In this case energy transferred is the Potential energy you gain.
So how (what equation) do you work out the power output?
Power = m x g x h
How to measure the power output of a person:
A timed acceleration where you have the weight of the person, the time taken, and the distance covered.
In this case energy transferred is the kinetic energy you gain.
So how (what equation) do you work out the power output?
Power = 1/2 x m x V squared / time
What is the equation for momentum including units and then the symbol equation…?
Momentum (Kg m/s) = Mass (kg) x Velocity (m/s)
m = p/ v
In a ……. system the total momentum before the collision is the same as … the event. This is called the ……. .. ……..
closed, after
Conservation of momentum
If you want to work out the total momentum of two people after they collide what are the steps taken to work this out? (4) Direction Original M Total M Where to now?
- Chose which direction is positive,e .g moving to the right
- Find the total momentum before the collision = momentum of x + the momentum of y. = z m/s
- Find the total momentum after the collision, so their momentum together. So add their weights together, then work out their velocity, so if one is moving right at 2 m/s, the other left at 1.5 m/s their velocity to the right is 0.5 m/s.
- So divide their joint weight by their joint velocity and that = the momentum they have together as a reaction of their crash.
If you want to work out the speed a gun moves backwards after it fires, what are the steps taken to work this out? (4)
Direction
Original M
Total M
Where to now and how fast?
Try not to think about momentum too much. :)
- Chose which direction is positive,e .g moving to the right
- Find the total momentum before firing = 0 kgm/s
- The total momentum after firing , so it
= the momentum of the bullet + the mom’ of the gun
so (weight times vel’ of the bullet) + (e.g 1 kg gun x V)
V is what we want to find. - So velocity of the bullet + V = 0 (C of mom’)
so if the vel’ of the bullet was 1.5… 1.5 + V = 0
so V = -1.5 m/s, so the gun moves backwards this fast.
What do safety features in cars now try to do?
The longer it takes for momentum to … the smaller the …. on a body.
Slow you down over a longer period of time.
change, force
When a force acts on an object is causes a …. in …..
A larger force means a …. change …. ……….
So if someone’s …. changes very ……., the …. on the body will be very …., more likely to cause an injury.
change in momentum
faster, in momentum
momentum, quickly, forces, large,
What are cars designed to do with their kinetic energy in a crash?
If a car slows down really quickly the kinetic energy will be ……. very quickly which could be … for the people inside.
Covert it very safely
converted, dangerous
How do crumple zones protect passengers?
- Where are they?
- What do they do?
- How does this reduce the car’s kinetic energy?
- What do the zones do overall to the crash?
- At the front and back of a car
- Crumple on impact
- It is converted into other energy by the car body as it changes shape
- Increase the impact time, they decrease the force produced by the change in momentum
How do seat belts protect passengers?
- What do they do?
- How does this protect the passenger? by doing what?
- Where does some of the kinetic energy of the wearer go?
- Stretch slightly
- Increase the time taken for the wearer to stop moving, reduces the forces acting in the chest
- Absorbed by the seat belt stretching
How do side impact bars protect passengers?
- What are they?
- How does this protect the passenger? by doing what?
- Where does some of the kinetic energy crash go?
Strong metal tubes fitted into car door panels
They help direct the kinetic energy of the crash away from the passengers to other areas of the car such as the crumple zones
How do air bags protect passengers?
- What do they do for the passenger?
- Preventing what?
Slow you down more gradually
You hitting hard surfaces inside the car
Some new brakes …. energy instead of … it
store, wasting
Brakes do … against the … energy of the car.
When you apply the brakes …. is done.
The breaks reduce the ….. energy of the car by … it into …. and … energy.
But what happens to the energy in some new electric or hybrid car, to the energy from braking?
work, kinetic
work
kinetic, converting, heat and sound
it’s converted into useful energy like electric for powering the car.
What do regenerative brakes do?
How?
At the same time the motor acts as an …?
What is the advantage of regenerative brakes?
Use the system that drives the vehicle to do most of the braking
The brakes put the motors energy into reverse and the the energy pf the pads runs the motor backwards slowing the wheels.
Electrical generator, converting the kinetic energy into electrical stored as chemical energy in the car’s battery.
The store the breaking energy rather than wasting it.
What determines the power of a car engine?
The more powerful the more energy it ….. from … every …. so the faster its top …. can be.
The size and design
transfers from fuel every second, speed
Cars are designed to be …..
What does this mean for the shape of the car?
arodynamic
It’s shaped in a way that air flows very easily around it and smoothly past minimising the air resistance.
What are equal when a car reaches top speed?
So with less ….. ……….. to overcome the car can reach a higher ….. before the above happens to the car.
So aerodynamic cars have … … ……
When the resistive forces, friction and air resistance, = the top driving force provided by the engine.
air resistance, speed
higher top speeds.
