Mechanics Flashcards
Principle of moments
For a system in equilibrium, the sum of the clockwise moments = the sum of the anti-clockwise moments
Where does weight of an object act from?
It’s centre of gravity
Equilibrium definition
- The components of the forces in both of any two perpendicular directions are balanced
- Clockwise moment at any point = anti-clockwise moment at that point
At equilibrium, when clockwise = anti-clockwise moments, what happens to forces?
Sum of forces = 0
Equilibrium with tension situation, what does the weight equal?
W = 2 Tsin angle
Resolving on a slope; as F=ma, with no friction, what is acceleration?
F = ma, F = W sin angle
mgsin angle/m = a
Resolving on a slope; as F=ma, with friction, what is acceleration?
mg sin angle - F / m = a
Newton’s first law
Objects stay at rest or in uniform motion unless acted on by a non- zero resultant force
Newton’s second law
The force on a body is proportional to its rate of change of momentum
F = mv-mu/t
F directly proportional to m(v-u)/t
F = K m(v-u)/t
Relationship between acceleration, resultant force and mass
Acceleration is
- directly proportional to resultant force
- inversely proportional to its mass
a = k/m
a =kF
Acceleration is always in the same direction as what?
The resultant force
Newton’s third law
When a body exerts a force on another body, they exert forces on each other that are equal in magnitude and opposite in direction
Newton pairs are:
Of the same type of force
Equal in magnitude
Opposite in direction
Acting on two objects
When a lift is accelerating upwards or decelerating downwards (T is larger than mg)
F=ma
T - mg = ma
a = T-mg/m
t = ma + mg
When a lift is accelerating downwards or decelerating upwards (T smaller than mg)
F= ma mg-t = ma
mg-t/m = a mg-ma = t
Moment
force x perpendicular distance from pivot to line of force
SUVAT only works when…
If the acceleration is constant
What affects terminal velocity?
Frontal area
Shape
Mass
Inelastic collision
A collision in which KE is not conserved
Totally/perfectly inelastic: objects move off together w same direction and speed
Elastic collision
A collision in which KE is conserved (in real life these don’t exist but some gas particles and sub atomic particles are closest)
What is Force if perfectly elastic collision
v = u, so
F = -2mu/t
Force time graph for a hard boundary
Tall curve in the middle of graph (short time, large force)
Force time graph for a soft boundary
Shorter spread out curve (force spread out over larger time)
Area under a force time graph
Impulse or change in momentum
