Friction
Coefficients of Friction and F = ma
Formulas to remember
Static Friction fs = UsFn Kinetic Friction fk = UkFn
(force to start object moving)
Horizontal surface
fs = Usw fk = Ukw AND w = mg
A 1000 N car skids on a wet concrete road. If the road is horizontal, what is the friction force on the car?
A skier is being pulled along a horizontal surface at constant speed with a force of 50 N. What is the weight of the skier?
A 10 kg block of wood sliding on a horizontal wooden table is brought to rest. What is the force on the block of wood that caused it to stop?
A 53. kg block, slowed by friction, has an acceleration of -0.1 m/s2. What is the force of friction on the block?
When a driver slams on the brakes, the wheels can lock and the tires skid. Anti-lock braking systems (ABS) rapidly pump the brakes so the tires keep rolling without skidding. Table 1 shows coefficients of friction for rubber tires on dry asphalt.
Table 1: Rubber Tires on Dry Asphalt
| Type of Friction | Coefficient |
|---|---|
| Static (tire rolling, not skidding) | 0.90 |
| Kinetic (tire skidding) | 0.68 |
Which statement best explains why ABS usually stops a car in a shorter distance?
Challenge score: 0 / 6
C1. What is the magnitude of the skidding car’s deceleration?
C2. Substitute numerical values into a mathematical representation that can be used to determine the maximum static friction force on a 1500-kg car with ABS on dry asphalt. Then calculate the force.
C3. A car with ABS is traveling at 25 m/s when the driver brakes hard. Assuming the tires keep rolling and the maximum static friction acts the whole time, determine the stopping distance.
C4. A student makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response using a mathematical relationship.
C5. A loaded truck and an empty car with the same tires skid on the same road from the same speed. How do their skidding distances compare?
C6. A car company is deciding whether to include ABS as standard equipment on an inexpensive model. Identify one benefit and one drawback of this decision.