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
