AQA A-Level Physics Summary - Forces and Momentum

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Force and momentum
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Last edited time @January 7, 2024 6:06 PM

Momentum
Momentum is defined as:

where:
ρ = mv
ρ =momentum [kgms−1 ]
m =mass [kg]
v =velocity [ms−1 ]

Momentum is ALWAYS conserved in a a straight line.
When using momentum, you must treat it as having magnitude and direction. You
must also remember that the sign matters. This is because it is a vector quantity.



📌 Example

Two cars collide and stick
together moving off as one after
the collision as shown:




Momentum and Newton’s laws




Δ

Force and momentum 1

, Δρ
NII = F = ⟹ Δρ = F Δt
Δt
​




NIII = A FB =B (−F )A ​ ​ ​ ​




Δρ Δρ
⟹ A ( ) =B ( )
Δt B Δt A
​ ​ ​ ​ ​ ​




BUT ​




A (Δe)B
​ ​ = B (Δe)A ⟹
​ ​

A (Δρ)B
​ ​ =B (Δρ)A​ ​




This demonstrates conservation of linear momentum.
Impulse
Impulse → A change in momentum.

Any collision results in a change in momentum (impulse).
This means that if Δtis increased the force, F , required to produce Δρcan be
decreased.

This gives rise to the impulse equation, which is as follows:

Δρ where:
F=
Δt F =force acting to cause the
​




⟹ F Δt = Δρ change in momentum [N].

Δt =time for which the force acts
Δρ = mv − mu
[s].
Δρ =change in momentum [Ns].
m =mass [kg].
u =initial velocity [ms−1 ](so
mu =initial momentum)
v =final velocity [ms−1 ](so mv =
final momentum)


Ethical transport design
Crumple zones, airbags and seatbelts are all designed to increase Δt, this
results in a decrease in required F because Δt α F .
Crumple zones and airbags - use permanent deformation.




Force and momentum 2