Mechanics and Materials

Question 1

Equilibrium

Answer 1

At rest/constant velocity
No acceleration
No resultant force

Question 2

Moment

Answer 2

Force multiplied by perpendicular distance from pivot in line of action of force

Question 3

Principle of Moments

Answer 3

At equilibrium - sum of clockwise moments = sum of anti-clockwise moments

Question 4

Describe what is meant by centre of mass and state where the centre of mass of a uniform object lies

Answer 4

Point at which weight of an object acts and for uniform objects that is directly in the middle

Question 5

Moment of a couple

Answer 5

Force multiplied by perpendicular distance between lines of action of pair of equal forces acting in opposite directions

Question 6

Explain why objects topple over if tilted beyond their critical angle

Answer 6

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

Question 7

Suggest why a vehicle full of people is more likely to topple over a cliff than an empty vehicle

Answer 7

Centre of mass will increase producing a greater moment beyond the pivot of the vehicle

Question 8

Describe how to calculate the maximum angle an object can be tilted before it topples over

Answer 8

Align line of action of weight with pivot and using the dimensions of the object calculate the angle between the object and the ground

Question 9

State the assumptions made for calculations involving projectile motion

Answer 9

No air resistance

Only gravity is acting

Question 10

Explain why maximum range is reached at an angle of 45 degrees to the ground

Answer 10

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.

Question 11

Suggest why objects of similar shape and size hit the ground at the same time regardless of their mass

Answer 11

Acceleration due to gravity is constant

Question 12

Explain why driving force must increase to ensure a constant velocity of a vehicle at higher speeds

Answer 12

Air resistance increases at higher velocities so driving force must increase to ensure a constant resultant force

Question 13

Suggest why air resistance increases when velocity increases

Answer 13

Air particles hit object with a greater force so exert a greater force against it and more air particles hit per second

Question 14

Suggest how a parachute protects skydivers

Answer 14

• increases drag force so velocity decreases
• reduces change in momentum when skydiver hits ground
• results in smaller impact force

Question 15

Terminal velocity

Answer 15

Maximum velocity reached by an object when drag force equals force causing motion of object

Question 16

Newton’s First Law of Motion

Answer 16

Objects remain at rest or move at a constant velocity unless acted upon by a resultant force

Question 17

Newton’s Second Law of Motion

Answer 17

The rate of change of momentum is proportional to the resultant force acting on an object

Question 18

Newton’s Third Law of Motion

Answer 18

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

Question 19

Explain how crumple zones in vehicles work

Answer 19

Increase collision time so increase time taken for change of momentum so impact force is reduced

Question 20

Explain how air bags in vehicles work

Answer 20

Spread impact force over a greater area so less pressure on body

Question 21

Impulse

Answer 21

Change of momentum of an object when the object is acted upon by a force for an interval of time

Question 22

Conservation of Momentum

Answer 22

Momentum before = Momentum after if no external forces are acting

Question 23

State one similarity and one difference between elastic and inelastic collisions

Answer 23

• 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)

Question 24

Energy

Answer 24

Ability to do work

Question 25

Work

Answer 25

Product of force and distance moved in direction of force

Question 26

Hooke's Law

Answer 26

- extension of a spring is proportional to the force needed to extend it - up to limit of proportionality

Question 27

Power

Answer 27

Rate of energy transfer

Question 28

Density

Answer 28

Mass per unit volume

Question 29

Elastic Limit

Answer 29

Maximum amount a material can be stretched and still return to its original length without being plastically deformed

Question 30

State the difference between calculating total spring constant of springs in series and parallel

Answer 30

Parallel - sum of spring constants | Series - reciprocal of sums of reciprocals of spring constants

Question 31

Stress

Answer 31

Force per unit CROSS SECTIONAL area, where line of action of force is perpendicular to cross section

Question 32

Strain

Answer 32

Extension per unit length of a solid when deformed

Question 33

Young's Modulus

Answer 33

Ratio of stress to strain (measure of stiffness)

Question 34

Yield Point

Answer 34

Point where material weakens temporarily and stress decreases for as strain increases

Question 35

Ultimate Tensile Stress

Answer 35

Stress needed to break a solid material

Question 36

Suggest what happens to kinetic energy in an elastic collision between a stationary ball and a ball with velocity

Answer 36

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

Question 37

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

Answer 37

- rebounds with same velocity for elastic and less for inelastic - change in momentum is -2mv for elastic and -m(u+v) for inelastic

Question 38

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.

Answer 38

- 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)

Question 39

Describe an experiment to test if speed is constant

Answer 39

- 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

Question 40

Describe how to draw unloading line for a material that has been plastically deformed

Answer 40

- parallel line that originates from last point on curve | - touches x axis after 0

Question 41

Suggest why horizontal and vertical motion are independent of each other

Answer 41

- gravity only acts downwards | - assuming air resistance is negligible

Question 42

Suggest why air resistance in negligible in the vertical direction but not horizontally

Answer 42

- air resistance increases with speed | - speed is low vertically but high horizontally

Question 43

Explain why air resistance decreases protection range

Answer 43

- projectile decelerates horizontally | - less distance travelled

Question 44

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

Answer 44

- 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

Question 45

Describe how to calculate Young's Modulus of a wire and suggest ways to ensure accuracy of data

Answer 45

- 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

Question 46

Suggest what area under stress-strain graph represents

Answer 46

energy stored per unit volume in material

Question 47

Explain why using an inclined plane rather than free fall produces valid data when investigating motion of a falling object

Answer 47

- free fall is too quick | - no accurate method to time free fall

Question 48

Describe what is meant by limit of proportionality for force-extension graph

Answer 48

point beyond which graph is no longer linear