More on Newton’s Law A car on a local street (i.e. air drag is negligible)

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Presentation transcript:

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 = )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 fkfk

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 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