Momentum Flashcards
momentum is a … quantity
momentum is a vector quantity
what is the equation for momentum?
momentum (kgm/s) = mass (kg) x velocity (m/s)
M = m x v
rearrange the equation for momentum to make mass the subject
mass = momentum ÷ velocity
m = M ÷ v
rearrange the equation for momentum to make velocity the subject
velocity = momentum ÷ mass
v = M ÷ m
vector quantities have a … and a …
vector quantities have a magnitude and a direction
a car is moving at 15m/s in a northerly direction
it’s mass is 2,000kg
calculate the momentum of the car
M = m x v
= 2000 x 15
= 30, 000 kgm/s north
what is the equation for velocity?
velocity (m/s) = distance (m) ÷ time (s)
v = d ÷ t
which direction does momentum act in?
in the direction of the velocity
explain the statement: the momentum of an object is directly proportional to its mass and velocity
M = m x v
as mass and velocity increase or decrease the momentum will increase or decrease proportionally (by the same factor, or by the same proportion)
what does proportionally mean in directly proportional?
by the same factor, or by the same proportion
calculate the momentum of an 80kg man running 10m/s
M = m x v
= 80 x 10
= 800 kgm/s
calculate the momentum of a 50g ball moving at 80km/h
M = m x v
convert 80km/hour into m/s so x 1000 ÷ (60 x 60) = 80000 ÷ 3600 = 22.2
convert 50g into kg so 50 ÷ 1000 = 0.05
= 0.05 x 2.2
= 1.1 kgm/s
a 1300kg car has the momentum of 3900kgm/s acting North
calculate the velocity of the car
M = m x v so v = M ÷ m
= 3900 ÷ 1300
= 3m/s North
a hammer is moving at 15m/s and has a momentum of 11.25kgm/s
calculate the mass of the hammer
m = M ÷ v
= 11.25 ÷ 15
= 0.75kg
what is the principle of the conservation of momentum?
the principle of the conservation of momentum states that the momentum is always conserved in a collision, provided there are no external forces (e.g. air resisitance, friction) acting on the system
give two examples about external forces that could disrupt the principle of the conservation of momentum
air resistance
friction
how does this illustrate the conservation of energy?
Before, the velocity is zero. Therefore, the momntum of the dynamte is zero as M = m x v. After, the dynamite explodes in the exact opposite direction (positive and negative). The equal positive and negative momentum cancel each other out having a total momentum of zero. This proves the conservation of momentum as the total momentum before must, and does, equal the total momentum after.
A ball which has 5kg mass and is moving east at 10m/s which knocks into another stationairy ball with the mass of 5kg.
Calculate the momentum before and after and calculte the velocity of the balls after
total momentum before = total momentum after
MaVa + MbVb = (Ma +Mb)V
(5 x 10) + (5 x 0) = (5 +5)V
50kgm/s = 50 ÷ (10) = V
5m/s = V
A ball which has 5kg mass and is moving east at 10m/s which collides with another ball with the mass of 5kg moving 5m/s west.
Calculate the momentum before and after and then calculat the velocity after
CHECK THE SHEET FOR THIS
total momentum before = total momentum after
MaVa - (backwards direction) MbVb = MaVa + MbVb
(5 x10) - (5x5) = (5 x 2) + (5 x V)
25kgm/s = 10 + (5 x V)
5 x 3 = 15, + 10 = 25
V = 3m/s
how do you convert milliseconds into seconds?
÷ 1000
how do you convert millimetres into metres?
÷ 1000
Use the conservation of momentum to calculate the recoil velocity of the gun:
mass of gun = 1.00kg
mass of bullet = 0.05kg
before: gun and bullet stationairy
after: gun recoils backwards, bullet moves fowards (east) at 300m/s
MgunVgun + MbulletVbullet = MgunVgun + MbulletVbullet
(1.00 x 0) + (0.05 x 0) = (1.00 x ?) + (0.05 x 300)
0 + 0 = (1.00 x ?) + 15
after momentum must equal to 0 as before momentum equals 0
15 ÷ 1.00 = 15
therefore, recoil velocity of the gun = 15 m/s backwards (west)
The recoil velocity of a 1.00kg gun is 15 m/s backwards (west)
Explain what would hapen to the recoil velocity if the gun was held by a 99kg person?
the velocity would decrease to x100 smaller as the mass of the gun is now 100kg (1.00kg + 99kg)
the recoil velocity is now 0.15m/s
What equation connects force, change in momentum and time taken for change to happen?
force acting (N) = change in momentum (kg m/s) ÷ time taken for change to happen (s)
What is Newton’s third law?
if object A exerts a force on object B then object B exerts the excat opposite force on object A
every action has an equal and opposite reaction
What does “every action has an equal and opposite reaction” mean?
The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second objec
Newtons Second Law (F = ma) to explain how safety features reduce the impact focus in a car crash?
seatbelts, airbags and a cripple zone increase the time taken for a change in momentum so that the impact force decreases
you cannot reduce the change in momentum - it is fixed
the only way to make the force smaller is to make the time bigger as this is an inverse equation
N.B. all always quote and reference this equation
What is Newton’s second law?
impact force = (final momentum - initial momentum) ÷ time for change
F = (mv - mu) ÷ t
F = m(v-u) ÷ t
(v-u) ÷ t = acceleration
F = ma
force = mass x acceleration
