# Dynamics

• Jun 27th 2009, 01:19 PM
Anenga
Dynamics
The UK Highway Code gives a stopping distance (after the brakes have been applied) of 75
m at 70 mph (31.1 m/s).
i) Draw a free-body diagram of a decelerating car [2]
ii) For the above stopping distance, calculate the rate of deceleration [4]
iii) What friction factor (coefficient of kinetic friction) has been assumed in calculating
these stopping distances?

its question part (iii) that am stuck at. anyone know?

Also,

Consider a crate of mass 1000kg sitting on a slope as shown in figure 1.
http://nintendodaft.com/images/slope.jpg
v) Draw the free-body diagram, assuming the block is kept stationary by friction and
determine the coefficient of static friction necessary if the block is kept stationary. [5]
vi) If the coefficient of kinetic friction is 0.1, what speed will the block be travelling at by
the time it reaches the bottom of the slope if it is given an initial nudge to overcome
static friction (sometimes called “sticktion”) [5]
vii) If the block is to be halted within 20m in the level ground after leaving the slope, what
retarding force will be necessary [5]
viii) What will be the required coefficient of kinetic friction?

Its vi, vii and viii am stuck at could someone help? cheers
• Jun 28th 2009, 02:14 AM
CaptainBlack
Quote:

Originally Posted by Anenga
The UK Highway Code gives a stopping distance (after the brakes have been applied) of 75
m at 70 mph (31.1 m/s).
i) Draw a free-body diagram of a decelerating car [2]
ii) For the above stopping distance, calculate the rate of deceleration [4]
iii) What friction factor (coefficient of kinetic friction) has been assumed in calculating
these stopping distances?

its question part (iii) that am stuck at. anyone know?

Frictional force is $\mu\, m\, g = |a|\, m$, where $|a|$ is the absolute value of the average decelleration.

CB