Momentum
Easy Peasy Stuff
• Momentum is a vector quantity i.e. it has a
magnitude and a direction.
• Momentum is conserved in collisions
i.e. momentum before collision =
momentum after.
• Momentum = mass x velocity
• Measured in kgms-1 or Ns
Keys to success are….
Recognising that it is a
momentum question
(Not too difficult
because it will have
two things colliding)
And drawing a good
diagram
Typical Question
Draw a diagram
Before
After
3m/s
0m/s
A
0.05kg
B
0.2kg
v
v
What do we know?
Momentum before = momentum after
Momentum before = 0.05 x 3 + 0.2 x 0
= 0.15
Negative because moving
In opposite direction
Momentum after
= 0.05 x (-v) + 0.2 x v
= 0.15v
Equating gives us
0.15 = 0.15v
so v = 1 ms-1
Works in 2-D as well
Don’t need to overly worry about
the direction as it is ‘taken care of’
by the vector
I’ll show you what I mean
Diagram
Before
0
0
A
0.3 kg
X
y
After
B
0.5kg
Using Conservation of Momentum
Mom. before = 0.3
=
2.1
1.2
=
2.1
1.2
+ 0.5
+ 0
0
0
0
mom. after = 0.3 x
y
+
= 0.3x
0.3y
=
0.3x + 3
0.3y
0.5
+
3
0
Equating gives us……..
2.1
1.2
so
=
0.3x + 3
0.3y
2.1 = 0.3x + 3
1.2 = 0.3y
velocity of A = -3
4
x = -3
y=4
b) find the speed of A
Speed is the magnitude of the vector so a bit
of Pythagoras’ Theorem gives us a speed
of ……..
-1
5ms
c) which is faster A or B?
Using Pythagoras’ Theorem again gives the
speed of B to be………
-1
6ms
So B is moving faster than A