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.

• 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

w

fk

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 towedSuppose 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 towedSuppose 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