Mechanics and Materials Flashcards
Equilibrium
At rest/constant velocity
No acceleration
No resultant force
Moment
Force multiplied by perpendicular distance from pivot in line of action of force
Principle of Moments
At equilibrium - sum of clockwise moments = sum of anti-clockwise moments
Describe what is meant by centre of mass and state where the centre of mass of a uniform object lies
Point at which weight of an object acts and for uniform objects that is directly in the middle
Moment of a couple
Force multiplied by perpendicular distance between lines of action of pair of equal forces acting in opposite directions
Explain why objects topple over if tilted beyond their critical angle
Objects topple over when the line of action of their weight passes beyond the pivot and a moment is produced about the pivot causing it to topple over
Suggest why a vehicle full of people is more likely to topple over a cliff than an empty vehicle
Centre of mass will increase producing a greater moment beyond the pivot of the vehicle
Describe how to calculate the maximum angle an object can be tilted before it topples over
Align line of action of weight with pivot and using the dimensions of the object calculate the angle between the object and the ground
State the assumptions made for calculations involving projectile motion
No air resistance
Only gravity is acting
Explain why maximum range is reached at an angle of 45 degrees to the ground
At 45 degrees the vertical component for velocity equals the horizontal component. This ensures the longest time of flight for the highest horizontal velocity in order to achieve maximum range.
Suggest why objects of similar shape and size hit the ground at the same time regardless of their mass
Acceleration due to gravity is constant
Explain why driving force must increase to ensure a constant velocity of a vehicle at higher speeds
Air resistance increases at higher velocities so driving force must increase to ensure a constant resultant force
Suggest why air resistance increases when velocity increases
Air particles hit object with a greater force so exert a greater force against it and more air particles hit per second
Suggest how a parachute protects skydivers
- increases drag force so velocity decreases
- reduces change in momentum when skydiver hits ground
- results in smaller impact force
Terminal velocity
Maximum velocity reached by an object when drag force equals force causing motion of object
Newton’s First Law of Motion
Objects remain at rest or move at a constant velocity unless acted upon by a resultant force
Newton’s Second Law of Motion
The rate of change of momentum is proportional to the resultant force acting on an object
Newton’s Third Law of Motion
If an object A exerts a force on object B, then object B must exert a force of equal magnitude and opposite direction back on object A
Explain how crumple zones in vehicles work
Increase collision time so increase time taken for change of momentum so impact force is reduced
Explain how air bags in vehicles work
Spread impact force over a greater area so less pressure on body
Impulse
Change of momentum of an object when the object is acted upon by a force for an interval of time
Conservation of Momentum
Momentum before = Momentum after if no external forces are acting
State one similarity and one difference between elastic and inelastic collisions
- momentum is conserved in both
- kinetic energy is conserved in elastic but not in inelastic (kinetic energy of object colliding is same before and after collision)
Energy
Ability to do work
Work
Product of force and distance moved in direction of force
Hooke’s Law
- extension of a spring is proportional to the force needed to extend it
- up to limit of proportionality
Power
Rate of energy transfer
Density
Mass per unit volume
Elastic Limit
Maximum amount a material can be stretched and still return to its original length without being plastically deformed
State the difference between calculating total spring constant of springs in series and parallel
Parallel - sum of spring constants
Series - reciprocal of sums of reciprocals of spring constants
Stress
Force per unit CROSS SECTIONAL area, where line of action of force is perpendicular to cross section
Strain
Extension per unit length of a solid when deformed
Young’s Modulus
Ratio of stress to strain (measure of stiffness)
Yield Point
Point where material weakens temporarily and stress decreases for as strain increases
Ultimate Tensile Stress
Stress needed to break a solid material
Suggest what happens to kinetic energy in an elastic collision between a stationary ball and a ball with velocity
All kinetic energy from one ball is transferred to the other so the ball with velocity initially comes to rest and the other ball moves off with the same velocity
A ball is bounced against the wall - compare its velocity and change in momentum as it rebounds off the wall for an elastic and inelastic collision
- rebounds with same velocity for elastic and less for inelastic
- change in momentum is -2mv for elastic and -m(u+v) for inelastic
A ball moves towards the boot of a footballer with some velocity and he kicks it. Discuss the consequences if the ball had approached the boot at a higher velocity but received the same impulse.
- initial momentum of ball greater
- same change in momentum
- initial and final momentum in opposite directions so final velocity of ball is lower since mv–mu = m(v+u)
Describe an experiment to test if speed is constant
- mark on equal distances along height object is dropped from
- measure time taken to travel each distance
- should be equal if constant speed
Or - place card on trolley
- card obscures light gate
- light gate attached to data logger and there should be zero acceleration between gates
Describe how to draw unloading line for a material that has been plastically deformed
- parallel line that originates from last point on curve
- touches x axis after 0
Suggest why horizontal and vertical motion are independent of each other
- gravity only acts downwards
- assuming air resistance is negligible
Suggest why air resistance in negligible in the vertical direction but not horizontally
- air resistance increases with speed
- speed is low vertically but high horizontally
Explain why air resistance decreases protection range
- projectile decelerates horizontally
- less distance travelled
Describe method by which mass of an object can be determined using a spring and suggest ways in which accuracy/safety of experiment could be improved
- measure initial length of spring with a metre rule
- apply different standard masses to spring one at a time and measure new length of spring
- extension = new length - original length of spring
- plot graph of mass (or F but relationship is proportional so either is accepted) against extension
- hang object of unknown mass from spring and measure extension
- interpolate mass of spring from graph
- clamp stand to bench/counterweight
- use set square to measure length of spring parallel
- confirm mass of standards with scale
Describe how to calculate Young’s Modulus of a wire and suggest ways to ensure accuracy of data
- measure original length with metre rule
- measure diameter of wire using micrometer
- measure extension with vernier calliper
- extension = new length - original length of wire
- suspend from wire from a height and apply standard masses then determine extension
- plot graph of extension against length and calculate gradient= e/F
- divide L/A by e/F to find Young’s Modulus of wire
- repeat
- measure diameter of wire at several places
- length of wire greater than a metre
- use set square to measure extension
- Searles control wire to negate temp change
Suggest what area under stress-strain graph represents
energy stored per unit volume in material
Explain why using an inclined plane rather than free fall produces valid data when investigating motion of a falling object
- free fall is too quick
- no accurate method to time free fall
Describe what is meant by limit of proportionality for force-extension graph
point beyond which graph is no longer linear
