Topic 1B - Momentum and Forces

Question 1

Define momentum

Answer 1

The tendency of an object to stay in motion

Question 2

Derive Impulse

Answer 2

F = ma (N2)
F = m (v/t)  (acceleration = change in velocity over time)

m (change in) v = Chnage in momentum

F = Change in p / t
Change in p = F t

Question 3

Derive the Law of Conservation of Momentum

Answer 3

F1 = -F2 (N3)
change p1 / t1 = change in p2 / t2 (impulse)

t1 = t2

Change in p1 = change in p2
m1(v1 - u1) = -m2(v2 - u2)
m1v1 - m1u1 = -m2v2 + m2u2
m1u1 + m2u2 = m1v1 + m2v2

total initial momentum = total final momentum
TF, momentum is conserved

Question 4

State Law of Conservation of Momentum in words

Answer 4

In an isolated system, the total initial momentum equals the total final momentum regardless of any interactions within the system.

Question 5

Step for using multi-image diagrams

Answer 5

1. identify mass of each object
2. draw lines for each object - (in proportion to mass e.g. if one mass is three times larger cover three times more time intervals)
3. Draw initial and final and labelled with distance
4. show that initial and final are the same

REMEMBER TO LABEL EVERYTHING

Question 6

explain rocket propulsion in terms of conversation of momentum

Answer 6

Releasing gas or ionised particles in one direction causes the rocket’s body to move in the other direction to maintain/conserve momentum

EQUATION
pf = pi
pf = 0 kgms
0 = m1v1 + m2v2
-m2v2 = m1v1

Question 7

Explain which solar sail would experience a greater acceleration:

1. 100% absorption of photons
2. 100% reflection of photons

Answer 7

100 % absorption:
pi (total) = pf (total) (LOCM)
pi (photon) + 0 (p of sail) = 0 (final of photon) + pf (sail)
pf (sail) = pi (photon)
TF, the final momentum of absorbing sail

100% reflecting:
pi = pf (LOCM)
pi = pf (photon) + pf (sail)
pi = -pi + pf (sail)
pf (sail) = 2pi
TF, the final momentum of the reflection sail

Momentum is proportional to acceleration
TF, the reflecting sail will have greater acceleration as it has a greater change in momentum.