Mechanics Flashcards
Scalar examples
Power Pressure Time Temperature Volume Distance Energy Speed
Vector examples
Weight Velocity Force Displacement Momentum
How do you give the direction of a resultant force
Angle
What do you do to find the resultant force if forces aren’t perpendicular
Tip-to-tail scale drawing
If a force system is in equilibrium and has two unknown lengths how do you find them
Tip to tail scale drawing
Vectors form a closed shape
No resultant force
How can you solve a system in equilibrium
Tip to tail scale drawing
Equating horizontal and vertical components
Conditions for equilibrium
No resultant force
No resultant moment
What is equilibrium
When there is no resultant force
No resultant momentum
So a stationary object stays stationary or moving object stays moving with a constant velocity
Mass vs weight
Mass is scalar so only has magnitude whereas weight is a vector so has magnitude and direction
Mass is the amount of material in an object whereas weight is the force exerted by gravity on an object
What is the centre of mass
The point through which all the weight is deemed to act
Through which the resultant moment is zero
What is toppling
If the weight vector of an object extends outside of its base it will topple
What happens if an objects weight is exactly on the borderline of the base
Neither topples or returns to its original position
How do you calculate the maximum angle before toppling on an inclined plane
Draw a diagram
Draw a line from centre of object diagonally to edge of corner closes to ground so touching the plane
The lengths of the triangle will be half the height and half the base length/radius
tan-1(radius/half height)
What is a moment
The four multiplied by the perpendicular distance from the line of action of the force to the pivot point
Measured in Nm
Moment formula
M=Fd
Perpendicular distance
Principle of moments
Sum of clockwise moments about any point is the same as the sum of anticlockwise moments about the same point for a body in equilibrium
Uniform
Centre of mass is its physical centre
Consistent density
Couple
A pair of equal and opposite coplanar forces
Which do not act along the same line of action
Torque
Moment of a couple: force multiplied by the perpendicular distance between the lines of action of the forces
Distance
Length of path travelled from start to end
Displacement
Straight line length in a given direction from start to end point
Velocity
Rate of change of displacement
Acceleration
Rate of change of velocity
What is earth’s velocity over a year
0ms²
Circle
Start is the end
Displacement is zero
Gradient of a displacement time graph
Velocity since s/t=v
Gradient of a velocity time graph
Acceleration since v/t=a
How does acceleration act in free fall
Vertically downwards
Suvat that isn’t given
s=vt-1/2at^2
When do you use a suvat
Constant acceleration
When do you use speed=distance/time
Constant velocity
Examples of free fall
Object dropped or thrown down
Object thrown upwards
An object projected in any direction as long as it is not touching anything or any forces acting on it
First thing to do before attempting suvat
Define positive direction
Why does the horizontal component of velocity stay constant
There is no horizontal acceleration
If acceleration is zero then velocity remains constant
Air resistance is negligible so only experiences force of weight vertically downwards
When is velocity zero at the start of projectile motion questions
NEVER
When is velocity zero at the end of projectile motion questions
NEVER
When catching a cricket ball a cricketer moves his hand a short distance in the direction of travel of the ball as it makes contact with his hand. Why
There is a larger distance
So the time taken for the momentum to reach zero is increased
If a=v/t, if t increases acceleration decreases
From f=ma, decreasing a decreases the force on his hand
What is Newton’s 1st Law of motion
If no resultant force is acting on an object
It will either remain at rest
or continue moving with a constant velocity
What is Newton’s 2nd Law of motion
The rate of change of momentum (acceleration) of a body is directly proportional to the resultant force acting on it
How is acceleration related to momentum
Rate of change of momentum
What is Newton’s 3rd Law of motion
When two objects interact they exert an equal and opposite force on each other
The forces are of the same type
What is tension
A pulling force carried by ropes or cables when taught
Where does tension act
Away from the points of contact
What is free fall
I dk check mbook
Why can’t you use suvats for terminal velocity
Velocity is not constant
What is terminal velocity
The constant velocity achieved by an object when its drag force is equal in size to the driving force but acting in opposite directions
What factors can affect the size of drag
Velocity
Cross sectional area
Fluid density
Shape of object
Why does an object reach terminal velocity
Increased collisions with air particles as velocity increases
So drag increases
Until eventually equal in size to the driving force but in opposite direction
Terminal velocity; start of jump
Vertical component of velocity is 0
0 air resistance
Fres is weight
Since F=ma, a=9.81ms^-2
Terminal velocity; short time after jump
Vertical component of velocity increases Air resistance increases Resultant force has decreased Acceleration less than 9.81 Continues to speed up but at a lesser rate
Terminal velocity; eventually (no parachute)
As velocity continues to increase so does air resistance
Terminal velocity reached when weight=air resistance
Acceleration is 0
Resultant force is 0
Continues at this speed
Terminal velocity; parachute opened
Air resistance increases to be larger than the weight
Resistance force acts upwards
Decelerate
Velocity decreases
Terminal velocity; after parachute opened
Velocity decreases so air resistance decreases
Fres decreases
Deceleration decreases
Slow down at a lesser rate
Terminal velocity; eventually (parachute)
Velocity continues to decrease until weight=air resistance= terminal velocity Acceleration is 0 Resultant force is 0 Continues at this speed Lower terminal velocity
Explain how the resultant force on a car changes as speed increases
As speed increases, air resistance increases
So acceleration would decrease
Resultant force decreases because F=ma
Explain using Newton’s laws of motion why a vehicle has a maximum speed
When the car and the air interact they exert an equal and opposite force on each other (3rd)
As speed increases, air resistance increases
So acceleration would decrease
Resultant force decreases because F=ma
Resultant force = 0
Means car will continue moving at this constant terminal velocity (1st)
Units for momentum
kgms^-1
Ns
Momentum equation
mass x velocity
p=mv
Equation derivation from N2nd law
Change of momentum/time=Resultant force
mv-mu/t=Fres
m(u-v)/t=Fres
Fres= vΔm/Δt
How do you handle flow rate questions
Consider the fluid after 1 second
Impulse
Force multiplied by the time for which the force acts
AKA change in momentum
Its a vector
Impulse equation
I=Δp=Ft
Area of a force time graph
Impulse
Principle of momentum
For a system of interacting objects the total momentum remains constant providing no external resultant forces act
(total before=total after)
Elastic vs inelastic collision
Elastic there is no loss of kinetic energy
Inelastic there is some loss of kinetic energy during the collision
Totally inelastic collision
Colliding objects stick together
How can an inelastic collision be possible if the law of conservation of energy applies
Momentum is always conserved in collisions
Total energy is always conserved in collisions
But kinetic energy may be converted to other forms e.g heat
Explosion
An event where two objects, initially at rest, fly apart f recoil with an equal and opposite amount of momentum
Energy is needed for what
Moving objects
Heating objects
Changing shapes of objects
Principle of conservation of energy
Energy cannot be created or destroyed
Only transferred from one type to another
Work done
Energy given to or removed by a force
Work done equations
W=Fs
Must be parallel distance so may have to use trig
For no loss of energy, energy is as such…
Ep at start=Ek at end
mgΔh=1/2mv^2
Work done against friction is equal to
The difference between initial and final energy
Power
The rate of transfer of energy
The rate at which work is done
Units for power
Watts
Power equations
Power=work done/time
Power=energy transferred/time
Power=force x velocity (Fres=0 and velocity is constant)
