Mechanics: Forces and Momentum

Question 1

What is momentum?
Units?

Answer 1

p = mass x velocity
Only changed when a force acts on it (N1L)
Units kgms^-1 or Ns

Question 2

If a moving object of constant momentum gains or loses mass..

Answer 2

its velocity will change to keep its momentum constant.

Question 3

Newton’s 2nd law of motion in terms of momentum?

Answer 3

The resultant force on an object is proportional to the the rate of change of its momentum.
F = Δ (mv) / Δ t = ma

Question 4

Impulse of a force ?

Answer 4

Impulse = force x time for which force acts = F Δt = Δ (mv)

Question 5

if a ball rebounds off a wall normally, what is the impact force?

Answer 5

Δmomentum = -mv -mu = -2mu
(assuming constant speed)

Impact force, F = -2mu/t
(the value is negative because the force on the ball is in the same direction as velocity after the impact)

Question 6

If impact is oblique, i.e. initial direction of ball not perpendicular to wall, the normal component of velocity used! If initial and final direction of ball at same θ to normal..

Answer 6

v = u,
so Δmomentum = -2mu cosθ

Question 7

Newton’s 3rd law of motion?

Answer 7

When two objects interact, they exert equal and opposite forces on each other.

Question 8

For the forces to be a force pair.. (N3L)

(5 points)

Answer 8

the two forces must be of same type and acting on different objects. they must have the same line of action. must be equal and opposite.

eg if weight and reaction force, NOT a force pair

Question 9

What’s the principle of conservation of momentum?

Answer 9

States that for a system of interacting objects, the total momentum remains constant, provided no external resultant force acts on the system.

So Σ final momentum = Σ initial momentum

Question 10

Head on collisions - if two objects of exactly equal and opposite momentums collide head on..

Answer 10

Σ final momentum = 0, and they will both stop.

Question 11

What is an elastic collision?

Answer 11

An elastic collision is one where there is no loss of Ek.

Question 12

What’s an inelastic collision?

Answer 12

Occurs when the colliding objects have less Ek after the collision than before the collision.

Question 13

For two or more objects that fly apart due to an explosion, their total Ek immediately after the explosion…

Answer 13

is less than the total chemical energy released in the explosion, because heat, light and sound all carry away energy.