MECHANICS AND MATERIALS

Question 1

Why is a lower centre of mass more stable?

Answer 1

It allows for greater angular displacement of the centre of gravity within the bounds of support.

(COM more likely to fall within base)

Question 2

Rule for 3 forces acting on a point for object to be in equillibrium

Answer 2

Sum of any two forces must equal the third.

Line of action of each force intersects the other at a single point (otherwise there would be net turning effect)

Question 3

How can the shape of a projectile cause a lift force

Answer 3

If the shape causes the air on top to flow faster than that on the bottom, there is lower pressure at the top so lift force is felt

Question 4

Effects of drag force

Answer 4

Acts in opposite direction to motion
Increases with speed
Reduces horizontal range
Reduces maximum height of projectile if projected above horizontal, and makes descent steeper than the ascent

Question 5

When is the tension in a lift greater/less than the weight

Answer 5

Greater - If lift is moving up and accelerating or if lift is moving down and decelerating

Less - If moving down and accelerating or moving up and decelerating

Question 6

When can f=ma not be used

Answer 6

When mass isn’t constant
In non inertial (accelerating/rotating) reference frames

Question 7

Energy transfers in free fall in a fluid

Answer 7

Potential energy transferred into Ek of fluid particles

Question 8

Resultant force of falling raindrop modelled by equation

Answer 8

C , ΣF=0, so constant is 0, just normal square root graph

Question 9

Shortcut for perfectly elastic collisions

Answer 9

Relative Velocity Of Approach (u1- u2) = Relative Velocity Of Separation (v2 - v1) [For perfectly elastic collisions]

Question 10

Which of the following statements relating to Newton’s third law is incorrect?

A The two forces are equal in magnitude
B The two forces are equal and opposite so there is no resultant force
C The two forces are of the same type
D The two forces are equal and opposite but do not cancel

Answer 10

Obvs B

Question 11

Which of the following statements relating to Newton’s third law is incorrect?

A The two forces are equal in magnitude
B The two forces are equal and opposite so there is no resultant force
C The two forces are of the same type
D The two forces are equal and opposite but do not cancel

Answer 11

Obvs B

Question 12

Significance of constant velocity on Ek

Answer 12

No change in Ek at constant v. a = 0 so ΣF = 0 therefore no work being done on object

Question 13

What is the centre of mass of an object?

Answer 13

The point through which a single force has no turning effect

Question 14

How does a car move forwards

Answer 14

Friction between wheel and ground pushes back on the ground, which in turn pushes forward on the wheel

Question 15

Explain how wheels skid/slip

Answer 15

While braking, the rotational velocity = 0, however wheel still moves forward due to momentum

Under slip when rotational velocity&raquo_space;>linear velocity, occurs as the force being applied exceeds the frictional force.

Question 16

State 2 factors that affect the usefulness of hydroelectric power for electricity production

Answer 16

• locations for power stations are limited
  (e.g. by geographical and meteorological
  factors)
• continuous generation may not be
  possible (e.g. times of drought)
• environmental impact (e.g. population is
  displaced when reservoir is built, damage to
  wildlife habitats, visual intrusion)

Question 17

Describe energy changes in an object at terminal velocity

Answer 17

No change in Ek of falling object as v is constant, so ALL of GPE is doing work against overcoming frictional forces

Question 18

Alternate approaches to answering impact force questions (airbag on a driver)

Answer 18

Energy:
* air bag absorbs EK of the driver
* over a greater distance
* ∆ EK = force × distance
* so force is reduced
Effect of pressure:
* air bag increases contact area
* force on driver acts over a
larger area
* pressure (force per unit area)
on driver is reduced

Question 19

Assuming the power output from a wind turbine is proportional to the Ek of the wind, discuss what would happen if the wind speed decreased by 1/2

Answer 19

• 1/2 As much mass passing through the blade each second
  Ek ∝ v^2 so E reduced to 1/4 of original value x 1/2 = 1/8

Question 20

Factors affecting the horizontal force on a ship being pulled by a sail

Answer 20

Angle of cable to horizontal
Surface area of sail
Wind speed/Ek
Acceleration of ship

Question 21

Note when doing change in momentum

Answer 21

If velocity is in an opposite direction, add velocities

Question 22

What wld happen without seatbelts

Answer 22

would have continued to move forward as
the car slowed down and she would have
collided with the back of a front seat or with the
front windscreen of the car
The impact time would have been much less so
the impact force would have been much
greater.

Question 23

Discuss effects of reducing speed limit from 30mph to 2omph for road safety

Answer 23

The stopping distance would be much less at 20
mph than at 30 mph.
This is because the braking distance depends
on the square of the speed so it would be
reduced by more than half i.e. (20/30)2
.
Fewer accidents would occur because cars
travelling at 20 mph would stop in a much
shorter distance than cars at 30 mph.

Question 24

Where drag force on falling projectile arises from

Answer 24

Friction between layers of air near the surface where air flows over the surface

Question 25

Factors affecting top speed of vehicle

Answer 25

Engine power + shape
More streamline, lower drag force. For constant engine power
F=Fe-D so a = Fe-D/M

D increases with speed, and max speed reached when D=Fe, (terminal so a=0)

Question 26

Things affecting drag force

Answer 26

Shape
Speed
Viscosity
Faster object greater drag force

Question 27

eqs for thinking distance and braking distance

Answer 27

Thinking = v.reaction time
Braking = v^2/2a for constant a

Question 28

Friction w cars stuff

Answer 28

If driver accelerates too fast, wheels skid - as there is an upper limit to the amount of friction that can be provided by road

Same for decelerating - car goes uncontrollably forward
Friction between road and tyres prevent wheel spin(slip)

https://www.youtube.com/watch?v=1UD560RQ684

Question 29

Eq for calculating impact time

Answer 29

S=1/2(u+v)t

Question 30

Deriving conservation of momentum

Answer 30

2 Balz - do resultant force on both, then N3L says equal and opposite, so yeah

Fr = m(v-u)/t

Question 31

Elastic vs inelastic

Answer 31

Inelastic loss of Ek

Question 32

When energy is conserved in explosion

Answer 32

Usually Ek conserved, but in an explosion heat, light and sound all carry away energy

Question 33

Deriving kinetic energy eq

Answer 33

Constant force on object from v=0 to v=v , s = 1/2(u+v) = v/2
a = v-u/t = v/t

W = fs = 1/2mv^2

Doesn’t hold for speeds close to c as mass changes w velocity so

Question 34

When a powered object gains speed

Answer 34

Output force exceeds resistive force
Motive power = power wasted due to resistive + gain in Ek/s

Question 35

P=fv

Answer 35

For constant velocity and force

Question 36

Pulleys mechanical advantage

Answer 36

Force multipliers, consider single puller holding 1kg weight, tension required for rope is 0.5N so 0.5N for 1N weight

However in order to pull weight a distance of 1m, person has to pull total distance of 2m for each section of rope

Can verify using W=fs, pulley changes force required to move over a distance

Mechanical adv = number of ropes holding up weight

Also note the speed of the person pulling will be nx speed of n ropes, as greater distance travelled in same time

Differences in energy/s due to friction in bearings + energy must be supplied to raise weight

Question 37

Deriving eq for effective k in series

Answer 37

Assume springs are massless, not a bad assumption since usually weight»>weight of springs

Question 38

To stretch/compress a material

Answer 38

A pair of forces is required

Question 39

Stable vs unstable equilibrium

Answer 39

A body is said to be in unstable equilibrium if it does not regains its original position after being slightly disturbed by an external force

Question 40

I beg you don’t use suvat

Answer 40

When acceleration isn’t constant

Question 41

Explaining unloading curve that doesn’t return to its original shape

Answer 41

Plastic deformation has produced permanent extension/re-alignment
of bonds in material hence intercept non-zero
Gradient is same because after extension identical forces between
bonds