momentum, moments and static electricity

Question 1

how do seat belts protect people?

Answer 1

1. they stretch slightly
2. this increases the time it takes for the person to stop moving which decreases the rate of change of momentum
3. this decreases the force exerted on the person wearing the seatbelt

Question 2

how do the crumple zones of cars protect people?

Answer 2

1. they crumple on impact
2. this decreases the rate of change of momentum
3. this decreases the force exerted on the people in the car

Question 3

how do air bags protect people in cars?

Answer 3

1. they slow people down more gradually which decreases the rate of change of momentum
2. this decreases the force exerted on the people in the car

Question 4

what is the conservation of momentum?

Answer 4

the total momentum before an events equal to the total momentum after an event in a closed system

Question 5

what is the relationship between force, momentum and time taken?

Answer 5

force = change in momentum / time taken

Question 6

what is Newton’s third law?

Answer 6

when two objects interact, they exert equal and opposite force on each other

Question 7

what is the centre of gravity?

Answer 7

the theoretical point where all the mass of an object is thought to be concentrated

Question 8

what is the equation linking force, moment and perpendicular distance?

Answer 8

moment = force x perpendicular distance between the line of action to the pivot

Question 9

what are the potential dangers of electrostatic charges when refuelling?

Answer 9

1. when fuel flows through a filler pipe, the friction between the fuel and pipe can cause a build up of static charge
2. this can cause a spark, which may cause the fuel to ignite and cause an explosion
3. this is prevented by earthing the fuel tank so that the charge is carried from the tank to the earth

Question 10

how does an inkjet printer use electrostatic charges?

Answer 10

1. tiny droplets of ink are forced out of a fine nozzle, making them electrically charged
2. these droplets are deflected as they pass between two charged plates (one is negative one is positive)
3. the droplets are attracted to the oppositely charged plate and repelled from the plate with a like charge
4. the size and direction of the plates’ charges are changed so that each droplet of ink hits a different place on the paper

Question 11

how does a photocopier use electrostatic charges?

Answer 11

1. an image is projected onto a positively charged copying plate
2. the plate loses its charge in the light areas and keeps it in the dark areas (the text)
3. negatively charged black toner it attracted to the remaining positively charged areas
4. the toner is transferred to paper
5. the paper is heated so that the toner sticks

Question 12

how does painting cars use electrostatic charges?

Answer 12

1. positively charged paint is sprayed
2. the particles repel each other so spread out, creating a fine mist
3. the positively charged paint is attracted to the negatively charged or earthed car