15. Momentum

Question 1

what is the conservation of momentum?

Answer 1

• if two objects collide, forces acting on each one will be same size but in opposite directions
• these forces change the velocity and momentum of each object
• therefore, momentum lost by one object will be gained by the other
• total momentum is same/conserved

Question 2

what is the relationship between momentum (which is also a vector) mass and velocity?

Answer 2

momentum = mass x velocity
p = m x v
momentum is in kgm/s

Question 3

eg a bullet with mass 0.3 kg is fired with a velocity of 100m/s. calculate the momentum of the bullet and the momentum of the gun

Answer 3

1. mass x velocity = o.3 x 100 = 30 kgm/s
2. momentum of gun + momentum of bullet = 0, therefore momentum of gun = -30kgm/s

Question 4

what is the principle during collisions?

Answer 4

total momentum before collision = total momentum after collision

Question 5

what is an elastic collision?

Answer 5

• no kinetic energy is transferred from the system (it does not change before and after collision)

Question 6

what is an inelastic collision?

Answer 6

• kinetic energy transferred from the system as heat, sound and to deforming objects
  -colliding objects that stick together is an example of an inelastic collision

Question 7

what needs to be considered given that momentum is a vector?

Answer 7

direction (can be positive or negative!)

Question 8

what is the relationship between force and change in momentum?

Answer 8

force = change in momentum (kgm/s) /time (s)

when a force is applied, velocity changes -> momentum changes

Question 8

how to answer a change in momentum q, eg a ball that is 0.5kg is kicked from stationary to a velocity of 8m/s, kicker’s foot is in contact w ball for 0.1 s, what force does kicker use?

Answer 8

1. find initial momentum = o.5 x o=0
2. find final momentum = 0.5 x 8= 4
3. force = 4-0 / 0.1 = 40 N

Question 8

how to answer a change in momentum q, eg a ball that is 0.5kg is kicked from stationary to a velocity of 8m/s, kicker’s foot is in contact w ball for 0.1 s, what force does kicker use?

Answer 8

1. find initial momentum = o.5 x o=0
2. find final momentum = 0.5 x 8= 4
3. force = 4-0 / 0.1 = 40 N

Question 9

reducing force in car crashes

Answer 9

if the same change in momentum occurs in a longer amount of time, then a smaller force will act
force = change in momentum/time
features such as:
seatbelts
airbags
crumple zones
increase the amount of time that there is a large change in momentum, so the car occupants experience less force