2. Personal and Public Transport - mechanics

Question 1

what are the formulas for

1. Mechanical advantage
2. velcoity ratio
3. effciency

Answer 1

1. MA = load / effort
2. VR = d _(effort) / d _(load)
3. efficency = MA/ VR

Note:

1. efficiency should ALWAYS be less than 100%
2. d_e and dL may be diameter, radius, circumference, or distance, just make sure d_E and d_L are the same units (ratio is a ratio) FOR A BELT AND PULLEY

Question 2

Velocity ratio on a gears

Gear ratio = no of teeth on driven

no of teeth driving gear

Question 3

What is the VR of this system

Answer 3

VR = 17/49 X 168/337.5

= 0.17

Question 4

What is the formula for Friction

Answer 4

F= µN

F = friction force (N)

µ= coefficeint of friction (varies for differnt surfaces)

N = Normal reactional force

• It always opposes the direction of motion
• Independent of surface area

NB: Be very careful that one does not treat the weight’s sliding force (mgsinθ) as friction, nor ignore it.

Question 5

What is the angle of static friction/ Repose

Answer 5

• The angle of static friction (ϕ) is the angle to the normal reaction that the resultant of the Friction and the normal makes.
• When the limiting friction is used, this angle is known as the angle of repose (θR) and will equal the angle of inclination(θ).