Mechanics Flashcards
What is a vector quantity?
A physical quantity with magnitude and direction.
What is a scalar quantity?
A physical quantity with only magnitude.
Give three examples of scalar quantities.
- Distance
- Speed
- Mass
Give five examples of vector quantities.
- Displacement
- Velocity
- Acceleration
- Momentum
When can vector quantities be added together?
When the vectors are parallel or antiparallel.
How can resultant vectors be calculated?
- Scaled diagram
- Pythagoras
- Trigonometry
- Resolve components
How are vectors added using a scaled diagram?
- The vectors are drawn to scale and joined head to tail in the correct direction.
- The resultant vector joins the polygon.
What are the components of a vector?
The perpendicular pair of vectors.
How can vectors be added by resolving components?
- The vertical and horizontal components of the vectors are calculated.
- The perpendicular components are added and the resultant vector calculated.
What is the component parallel to the slope of a weight?
mgsinx
What is the component perpendicular to the slope of a weight?
mgcosx
Which forces acting on a weight are parallel to the slope?
- Friction
- Weight component
Which forces acting on a weight are perpendicular to the slope?
- Normal reaction force
- Weight component
What is a moment?
A force that produces a turning effect given by force x perpendicular distance from the line of action of the force to the pivot.
What is a moment also known as?
Torque
How can a moment be increased?
- Increased force
- increased distance
What is the principle of moments?
At balance, the sum of the clockwise moments about a point is equal to the sum of the clockwise moments about the same point.
What is the centre of gravity?
The point from which all the weight of an object appears to from.
Explain how the centre of gravity of a lamina can be found.
- Make a small hole in the corner of the shape and suspend it from a pin so it can move freely.
- Hang a plumb line from the pin and mark the vertical line.
- Repeat for two more points.
- The point of intersection shows the centre of gravity.
How can an object be made stable?
- Low centre of mass.
- Large base area.
When will an object tip over?
When the centre of gravity passes outside the base.
What is statics?
The study of forces acting on stationary objects.
What is Newtonian mechanics?
The study of objects that obey Newton’s three laws of motion.
Define coplanar forces.
Forces that all act in the same plane.
What happens when the vectors acting on a point form a closed polygon?
There is no resultant vector.
Give the two conditions for static equilibrium of coplanar forces.
- The resultant vector must be zero.
- The principle of moments must apply.
What is the principle of concurrency?
For three coplanar forces in equilibrium, the lines action must all pass though one point. (be concurrent)
What does the principle of concurrency apply to?
An object acted on by three forces.
Define dynamics.
The study of forces that may result in motion.
What happens if there is no resultant force on an object?
There is no change in velocity.
What is Newton’s first law?
An object will remain at rest or in uniform motion unless acted on by an unbalanced force.
What is Newton’s second law?
The resultant force is directly proportional to the rate of change of momentum.
What is Newton’s third law?
If object A exerts a force on object B, object B will exert an equal and opposite force on force A.
What are the two factors that affect the acceleration of an object?
- The size of the resultant force.
- The mass of the object.
What is relationship between resultant force, mass and acceleration?
F = ma
How is weight calculated from mass?
W = mg
What are the two forces acting on a falling object?
- Weight
- Air resistance
How does air resistance change with velocity?
Air resistance increases as velocity increases.
Describe the changes in velocity of a parachutist.
- There is an initial acceleration of g.
- Acceleration decreases until the terminal velocity is reached.
- Once the parachute is opened there is a large deceleration.
- Deceleration decreases until the terminal velocity is reached.
What are the two rules of Newton’s third law?
- The forces act on different objects.
- The forces must be the same type.
Which two forces act on an object sitting on the ground?
- Weight
- Reaction force
What causes a person to feel weightless?
When the reaction for is less than the weight.
How is average speed calculated?
distance travelled / time taken
What two things can the gradient of a distance vs. time graph show?
- Speed
- If the gradient is constant the speed is constant.
What does the area under a speed vs. time graph show?
Distance travelled
What does the gradient of a displacement vs. time show?
Velocity
What does a negative gradient on a displacement vs. time graph show?
The direction of travel has changed.
What does the area under a velocity vs. time graph show?
Displacement
What does the gradient of a velocity vs. time graph show?
Acceleration
Why is an object moving in a circle constantly accelerating?
- Acceleration is given by change of velocity.
- If the object is moving in a circle, the velocity is changing.
What are the four SUVAT equations?
s=([u+v]/2)t
v=u+at
s=ut+0.5at^2
v^2=u^2+2as
How is kinetic energy calculated?
1/2mv^2
How is GPE calculated?
mgh
How is change in GPE calculated?
mgΔh
How is velocity calculated from KE and GPE?
v=(2gh)^2
How is work done calculated?
force x displacement of the force
What is work done equivalent to?
Energy transferred
How is efficiency calculated?
(useful energy out / total energy in) x 100%
What is power?
The rate of energy transfer
How is power calculated?
energy transferred / time taken
or
force x velocity
How is momentum calculated?
mass x velocity
What is the unit of momentum?
kgm/s
What type of quantity is momentum?
Vector
What is the rate of change of momentum equal to?
The resultant force causing it.
What is the change in momentum of an object experiencing a force equal to?
The impulse acting on the object.
What is the unit of an impulse?
Ns / kgm/s
What does the area under a force vs. time graph give?
Change in momentum
How do crumple zones improve car safety?
The rate of change of momentum is reduced which reduces the force acting on the car.
What three car safety features utilise the fact that rate of change of momentum = force?
- Crumple zones
- Seatbelts
- Airbags
What is the law of conservation of momentum?
In any collision in which no external forces act, the total momentum before the collision is equal to the total momentum after the collision.
What is an elastic collision?
One in which kinetic energy as well as momentum is conserved.
What happens to a body in stable equilibrium when displaced then released?
It returns to its equilibrium position.
When will an object tilt?
When Fd>Wb/2
What three principles apply to all projectiles?
- The acceleration of the object is always equal to g.
- The horizontal velocity is constant.
- The motions in the horizontal and vertical directions are independent.
What happens to the path of a stone thrown from a cliff if the initial projection is horizontal?
- The path becomes steeper as it drops.
- The faster it is projected, the further away it will fall.
- The time taken for it to fall doesn’t depend on how fast it is projected.
What is rocket thrust equal to?
T=mg+ma
What three things does drag force depend on?
- Shape of the object
- Speed
- Viscosity of fluid.
What is thinking distance?
The distance travelled by a vehicle in the time it takes the driver to react.
How is thinking distance calculated?
s = speed x reaction time
How is braking distance calculated?
s = v^2/2a
How is stopping distance calculated?
vt + v^2/2a
How is impact time calculated?
t = 2s/u+v
How is impact force calculated?
F = change in Ek / impact distance
Give five car features that increase impact time.
- Bumpers
- Crumple zones
- Seat belts
- Collapsible steering wheel
- Airbags
How is work done calculated?
force x distance moved in the direction of the force
How is motive power calculated?
energy per second wasted + gain of kinetic energy per second
How is the power generated from a wind turbine generated?
0.5ρv^3A
How is the power generated from a hydroelectric power station generated?
P=ηρgh
η=efficiency
How much energy can 1m^2 of a solar panel absorb in space?
1400J/s
How is density calculated?
ρ=m/V
How is the density of an alloy calculated?
ρ = ρAVA/Vtotal + ρBVB/Vtotal
What is Hooke’s Law?
The force needed to stretch a spring is directly proportional to the extension of the spring.
How is the force needed to stretch a spring calculated?
F = kΔL
What is the unit of spring constant?
N/m
What happens when a spring is stretched beyond its elastic limit?
It never regains its original length when the force is removed.
What is the spring constant?
Force required to extend a spring by 1m.
What is the effective spring constant of two springs in parallel?
2k
What is the effective spring constant of two springs in series?
k/2
How is the effective spring constant of two springs in constant calculated?
1/k = 1/kp + 1/kq
How is the elastic potential energy stored in a stretched spring calculated?
0.5kΔL^2
What is the elasticity of a material?
Its ability to regain its shape after being deformed.
What can be used to measure the extension of a wire under tension?
Searle’s apparatus
How is tensile stress calculated?
σ = T/A
How is tensile strain calculated?
ε = ΔL/L
What is the Young modulus of a material?
The tensile stress divided by the tensile strain.
How is the cross section area calculated for a wire?
A = πd^2 / 4
What is the yield point?
The point at which the wire temporarily weakens.
What is a ductile material?
A material which exhumes plastic behaviour.
How is the elastic energy stored in a stretched wire calculated?
0.5TΔL
