Transcript Lecture 25 and 26, Nov 3, Nov 5, 2008

More on Newton’s Law
A car on a local street
(i.e. air drag is negligible)
Example: Braking a car
A car (m = 1000 kg) travels at 54 km/h on a
street when suddenly the car brakes lock
the wheels. The coefficient of kinetic
friction of the car’s tires on dry asphalts
is k = 0.6.
1) How long will it take for this car to stop?
2) How far will it move when it stops?
• Free body diagram of the car:
n
fk
w
Q1 (revisit)
Consider a box on the top of the car which
is slowing down. Can the box stay at rest
on the top of the car?
1) yes;
2) no;
3) Not enough information.
A car being towed
Suppose the car now is dead and towed away
by a truck. (textbook page 121, Fig 4.31).
The pulling force on the car (by the truck) is
A) bigger;
B) smaller;
C) Equal to;
D) Not enough information.
than the pulling force on the truck (by the car).
A car being towed
Suppose the car now is dead and towed away
by a truck. The tension along the cable
which connects the car and truck depends
on (no air resistance)
A) rolling friction forces on the car;
B) velocity of the car;
C) acceleration of the car;
D) Both A) and C)
E) Both A) and B)
Drag force in gases/liquids
Drag force increases with velocity
D= c A v^2;
A is the cross-section area of an object;
v is the velocity.
c is the drag coefficient.
(What is the right SI unit for ‘c’?)
For the air, c=1/4 in SI units.
Which area ?
Air drag versus friction
A typical passenger car
with rolling friction
coefficient 0.02.
At which speed does the
air drag become bigger
than the friction force?
Hint: The cross-section is
assumed to be 2m^2;
car weight is about
1500kg.
Skydiver
A skydiver jumps off a plane at 3000m.
He falls with his belly “facing down” to 1000m
altitude before opening up his parachute.
1) Draw, qualitatively, the velocity graph for a
skydiver.
2) Design a parachute.
Velocity graph for a free falling object