Caldervale High School
CfE Higher Physics
Our Dynamic Universe
Summary Notes (Section 3)
Section 3: Collisions, Explosions and Impulse
Momentum
We define the Momentum (p) of an object as:
Momentum
=
mass
x
velocity
Momentum is measured in ___________ .
Momentum is a _____________ ___________________ (size and direction)
The LAW OF CONSERVATION OF MOMENTUM is
“______________________________________________________________
______________________________________________________________
______________________________________________________________ .”
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Caldervale High School
CfE Higher Physics
Worked Example 3A – Objects Sticking Together after Collision
A trolley of mass 4kg travelling at 2 ms-1 collides and sticks to a 3kg stationary trolley.
Determine the velocity of the two trolleys after the collision.
Worked Example 3B – Objects Sticking Together after Collision
What would be the velocity of the two trolleys after the collision had the 3kg trolley
been moving towards the 4kg one at 2 ms-1?
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CfE Higher Physics
Types of Collision
An elastic collision is a collision in which _________________________________
__________________________________________________________________ .
This is very rare (eg. In Molecular collisions with no contact!)
An inelastic collision is a collision in which _______________________________
__________________________________________________________________ .
This is much more realistic! (eg. Car crash!)
Worked Example 4 – Objects Don’t Stick Together after Collision
Hannah’ car has a mass of 2000 kg and is moving at 10 ms-1 when it crashes into
Kyle’s stationary car which has a mass of 1500 kg. Both cars move off separately
after the collision.
Kyle’s car moves off at 8 ms-1. Determine the speed of Hannah’s car after the
collision.
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Caldervale High School
CfE Higher Physics
Example 5 – Explosions
A firework reaches it’s maximum height and explodes into two parts, moving in
opposite directions. One part has a mass of 0.5 kg and travels at 15 ms -1. The other
part has a mass of 1.5 kg. Determine it’s speed after the explosion.
Impulse (p)
During any collision some momentum is transferred from one object to another.
So… momentum of one object __________________ by p and momentum of the
other object ____________________ by p.
p =
p =
Note: Impulse is just “change in momentum” so has momentum units!
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Caldervale High School
CfE Higher Physics
If the velocity of one object before the collision is “u” and the velocity of the same
object, after the collision, is “v”, then
impulse =
Then consider that during the collision the object experiences an average force, F,
over a contact time, t, causing an acceleration, a.
p = m (v - u)
then
= m(
)
So
IMPULSE
=
p
=
m (v – u) = F t
Note: Impulse units can therefore also be “Ns”.
Worked Example 1
A 0.8 kg trolley travelling at 0.7 ms-1 hits a wall. The trolley rebounds at 0.3 ms-1.
a)
Calculate the impulse of the force on the trolley
b)
The trolley is in contact with the wall for 50 ms. Calculate the average force
the wall exerts on the trolley during this time.
.
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Caldervale High School
CfE Higher Physics
Impulse from a Force – Time Graph
A force-time graph can be plotted showing how the force on an object changes
during the contact time of a collision.
The equation, m (v-u) = F t, refers to average force! In reality, the force on an
object would vary like this:
So, as IMPULSE = F t ,
then
IMPULSE =
Worked Example 2
During a collision the force experienced by a stationary object was recorded as
follows:
Determine the speed of the object after the collision.
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Caldervale High School
CfE Higher Physics
Key Questions on Impulse
A car travelling at 20 ms-1 crashes into a wall and is brought to rest by the wall.
An identical car crashes into the same wall at the same speed, but it has a
cushioned bumper so it takes longer to come to rest
In which collision is the impulse greater? Explain your answer.
2.
Explain why the second car experiences less damage.
3.
On the one set of axes, sketch graphs of force against contact time for
each collision.
Force (N)
1.
0
time (s)
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Caldervale High School
CfE Higher Physics
Newton’s Third Law
From National 5 you might remember Newton’s Third Law …
“Every force has an equal and opposite reaction force.”
This law is the basis of Impulse theory.
Think outside the box ???!!!
Try to make the connection between Newton’s Third Law and Impulse theory.
Use the space below to show the connection.
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Caldervale High School
CfE Higher Physics