2 Mechanics Flashcards
velocity, v [m s-1]
rate of change of displacement with time
v = s/t
acceleration, a [m s-2]
rate of change of velocity with time
a = ∆v/∆t
(translational) equilibrium
resultant force = 0 → acceleration = 0
conserved
maintains a constant total value
eg. KE is conserved in elastic collisions; total mechanical energy is conserved when friction is negligible and KE/PE are not changed to other forms (such as sound, internal energy)
conservation of energy
energy can not be created or destroyed, it can only change form
Newton’s 1st law
a body will remain stationary or at constant velocity unless a net force acts on it
Newton’s 2nd law
F = ma
Newton’s 3rd law
every applied force has an EQUAL and OPPOSITE reaction force (same size, same type, opposite direction) eg. man on earth ⇔ earth on man)
impulse, Δp [kg m s-1] or [N s]
the CHANGE IN MOMENTUM of a body
law of conservation of momentum
For an ISOLATED system (no external force);
momentum before = momentum after (collision);
(m₁u₁) + (m₂u₂) = (m₁+m₂)(v)
work, W [J]
work done = energy transferred=force × distance moved in the direction of the force
W = ∆E = F * d
kinetic energy, KE [J]
energy due to MOTION
KE = ½ mv² = ½ m(v²-u²)
potential energy, PE [J]
energy due to RELATIVE POSITION
GPE = mg∆h
elastic collision
both MOMENTUM and TOTAL KE are conserved → KE before = KE after
terminal velocity
MAX velocity when forces are BALANCED
If v and a are in the same direction, the object…
accelerates
If v and a are in the opposite direction, the object…
decelerates
What is the area of a-t graph?
∆v
For projectile motion, MAXIMUM HEIGHT means?
t = t/2
v=0
Describe the effects of air resistance on a projectile (height, range, shape)
- Lower max height
- Shorter range
- Smaller acceleration downwards
Air resistance increases as velocity increases until reaching terminal velocity where drag=mg!!
when forces are balanced, the object is…
at rest or moving at constant speed
moving at CONSTANT SPEED means…
forces are balanced→ net force =0→ acceleration = 0
Explain how you make a bicycle go forward in terms of Newton’s 3rd Law:
tyres roll and push backward on the ground → ground exerts equal and opposite reaction force, pushing the tyres forward
If two objects hang over a pulley or are connected with a rope, it means…
Same T (tension)
Same a (acceleration)
If two objects travel in opposite direction, they have ____ for velocity/momentum
opposite signs!
Explain how conservation of momentum applies as a book falls towards the surface of the Earth.
Forces on E and B are equal and opposite (3rd Law)
∆P = F∆t → ∆P of E and B are also equal and opposite → P stays as zero
Explain how increased DEFORMATION of the golf club can increase the SPEED at which the golf ball leaves the club
increase deformation = increased DURATION of contact (∆t)
∆P = F∆t = m(v-u)
∆t increase → v increase
Explain why a rocket accelerates forward in terms of momentum and Newton’s Laws
CHANGE IN MOMENTUM of the gas gives rise to a FORCE on the rocket that pushes it forward (F=∆P/∆t = ma)
What is the properties of an explosion (eg. nuclear fission, bullet leave gun)?
momentum = 0
KE increase
With reference to forces, explain why a person riding bicycle travels in constant velocity.
Vertical : mg=2N
Horizontal: backward force of tyre on ground = forward force of ground on tyre (Newton’s 3rd law)
Resultant force F=0 → a=0 (Newton’s 2nd Law)
Explain why we feel ‘weightless’ while riding a rollercoaster.
we fall at the same rate as the rollercoaster → no resultant contact force
