Mechanics and material

Question 1

What is the moment of of force about a point?

Answer 1

The force x the perpendicular distance from the line of action of the force to the point.

Question 2

What is the principle of moments?

Answer 2

For an object in equilibrium, the sum of the clockwise moments about any point is equal to the sum of the anticlockwise moments about that point.

Question 3

What is the centre of mass?

Answer 3

It is the point through which a single force on the body has no turning effect.

Question 4

What is a couple?

Answer 4

Pair of equal and opposite coplanar forces acting on a body but not along the same line

Question 5

What is a projectile?

Answer 5

An object in motion acted on only by the force of gravity.

Question 6

Newton’s first law

Answer 6

An object remains at rest or in uniform motion unless acted on by a resultant force.

Question 7

Newton’s second law

Answer 7

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

Question 8

Inertia

Answer 8

Resistance of an object to change its motion

Question 9

What is Newton’s third law?

Answer 9

When two objects interact they exert equal and opposite forces on each other

Question 10

What is an inelastic collision?

Answer 10

When the colliding objects have less kinetic energy after the collision than before the collision.

Question 11

What is energy?

Answer 11

The capacity to do work

Question 12

What is kinetic energy?

Answer 12

The energy of an object due to its motion

Question 13

What is potential energy?

Answer 13

The energy of an object due to its position.

Question 14

Hooke’s law

Answer 14

The force needed to stretch a spring is directly proportional to the extension of the spring from its original length.

Question 15

How to find the effective spring constant when two springs are in parallel

Answer 15

Add the two individual spring constants

Question 16

How to find the effective spring constant when two springs are in series

Answer 16

1/k = 1/k1 + 1/k2

Question 17

Elastic limit

Answer 17

The point beyond which a wire is permanently extended so will not return to its original dimensions when the force applied to it is removed.

Question 18

How to calculate the elastic potential energy stored in a spring from a force against extension graph

Answer 18

The area under the graph

Question 21

Define elasticity

Answer 21

Property of a solid that enables it to regain its shape after it has been deformed or distorted.

Question 22

Define tensile stress

Answer 22

Force per unit area of cross-section in a solid perpendicular to the cross-section.

Question 23

Define tensile strain

Answer 23

Extension per unit length of a solid when deformed

Question 24

What is the Young’s modulus of a material?

Answer 24

Stress/strain (provided the limit of proportionality between extension and force hasn’t been exceeded. Used to measure the stiffness of a material.

Question 25

What is the yield point of a wire?

Answer 25

The point at which the stress suddenly drops when the wire is subjected to increasing strain

Question 26

Define plastic deformation

Answer 26

Deformation of a solid beyond it’s elastic limit

Question 27

What is ultimate tensile stress? (UTS)

Answer 27

Tensile stress needed to break a solid material. Used to measure the strength of a material.

Question 28

Force against extension graph for a metal wire

Answer 28

Straight line through zero for loading curve up to elastic limit. Unloading curve is same as loading curve if elastic limit isn’t reached. If it is, the unloading curve is the same but shifted to the right a bit.

Question 29

Force against extension graph for rubber

Answer 29

Loading: steep then shallow then steep.
Unloading: similar but underneath loading curve.

Question 30

Force against extension graph for polythene

Answer 30

Loading: steep then shallow then steep
Unloading: very steep straight line down

Question 31

Define brittle

Answer 31

Snaps without stretching or bending when subject to stress

Question 32

Define ductile

Answer 32

Stretches easily without breaking so can be drawn into wires.

Question 33

What is the moment of a couple?

Answer 33

Force (of one) multiplied by the perpendicular distances between the lines of action of the forces.

Question 34

How is the motion of two objects different immediately after an elastic or inelastic collision?

Answer 34

The objects will rebound in opposite directions. The velocity of the lighter one will be greater than the heavier one. The momentum of each after is the same as the momentum of the other before. Total momentum has same value and direction before and after

Question 35

Why is there a gap between the loading and unloading curve for rubber?

Answer 35

It represents the difference between energy stored when it is stretched and the useful energy recovered from when it is unstretched. The difference occurs because some of the energy stored becomes the internal energy (potential and kinetic) of the molecules when it unstretches.

Question 36

How to measure the value of g using motion sensor

Answer 36

Clamp motion sensor to a stand so it is over the edge of the table. Make sure it is pointing perpendicular to the floor. Connect it to a data logger. Drop an object from just under the motion sensor while it is recording data. Calculate gradient of velocity time graph (for when it is falling) produced on data logger to find the acceleration. This is g. Do from maximum height possible to reduce percentage error.

Question 37

How to find Young’s modulus of a wire

Answer 37

Clamp wire to desk and attach a marker to it a known length from where the wire emerges from the clamp at 0 tension. Attach mass holder to end of wire over a pulley with the marker on the opposite side of the pulley. The weight of the masses is the force on the wire. For each new weight, find distance between clamp and marker to find extension. Use gradient of load against extension graph, cross-sectional area and original length to find YM. Between each result, remove masses and check wire returns to original length and haven’t exceeded elastic limit.