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Chapter 5
Momentum &
Impulse
Terms
Predicting motion requires us to understand
motion.
Useful terms
• Acceleration
• Force
• Impulse
Acceleration
Acceleration is the rate at which an object changes
speed.
It is measured in units of speed per units of time
𝑚𝑝ℎ
60 miles per hour per 4 seconds, 60
4𝑠
Every four seconds, the speed increases
by 60 mph
9.8
𝑚
𝑠
9.8 meters per second per second,
𝑠
Every second, the speed increases by
9.8 m/s
Acceleration
Acceleration can be expressed symbolically
∆𝑣
𝑎=
∆𝑡
Where
𝑎 = acceleration
∆𝑣 = change in velocity
∆𝑡 = change in time
Forces
Force is a push or a pull. When forces are
unbalanced, they cause acceleration.
Measured in Newtons or pounds
1 Newton ~ weight of 1 apple on Earth
1 pound ~ 4.5 N
Acceleration & Force
The amount of acceleration experienced by an
object is proportional to the amount of force
acting on it.
bigger force  bigger acceleration
The amount of acceleration is inversely
proportional to the mass of the object.
bigger mass smaller acceleration
Acceleration & Force
The relationship between acceleration, force,
and mass can be represented symbolically:
𝐹
𝑎=
𝑚
Where 𝑎 = acceleration
𝐹 = net force on object
𝑚 = mass of object
Not-so-secret universe
𝑎=
∆𝑣
and
∆𝑡
𝑎=
Therefore
∆𝑣 𝐹
=
∆𝑡 𝑚
and
𝑚∆𝑣 = 𝐹∆𝑡
𝐹
𝑚
And so…
𝑚∆𝑣 = 𝐹∆𝑡
To change an object’s speed or direction
requires force to be exerted on that object.
• Quick changes in speed require lots of force
• Long changes in speed require less force.
Think about transportation safety devices.
How do these devices take advantage of this principle?
When the change in your velocity is from something
large to zero (e.g., if you are in a car crash), you want
the force acting on you to be small. Airbags extend the
time.
When you are trying to karate chop a wood block in
half (i.e., when you change the velocity from zero to
something), you want the force to be large. A quick,
abrupt motion shortens the time.
Impulse
Impulse, often abbreviated 𝐼, is a measure of
the length of time, ∆𝑡, that a force, 𝐹, is applied.
𝐼 = 𝐹∆𝑡
Reported in units of Newton  seconds.
How much?
How much?