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Physics For Scientists & Engineers
Found in: Page 138

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Short Answer

A car is moving forward slowly and is spending up. A student claims that “the car’s engine exerts a force on itself” or that “the car’s engine exerts a force on the car.”

(a) Argue that this idea cannot be accurate and that friction exerted by the road is the propulsive force on the car. Make your evidence and reasoning as persuasive as possible.

(b) Is it static or kinetic friction?

Suggestion: Consider light gravel. Consider a sharp print of the tire tread on an asphalt road, obtained by coating the thread with dust.

If a car is stuck in on an icy road, with its wheel spinning wildly, you know the car has great difficulty moving forward until it catches on a rough patch.

See the step by step solution

Step by Step Solution

Step1: A concept:

Friction is the force that prevents the tire rubber from sliding on the road surface. However, things are not really that simple. Consider two different frictions; static and kinetic friction. Static Friction - the force of friction between surfaces that are not moving relative to each other.

Step 2: (a) The road's friction propels you forward:

The force that causes the car to speed ahead is friction from the road. Burning gasoline can provide energy for the motion, but only external forces- forces exerted by objects outside can accelerate the car.

Step 3: (b) It is a case of static friction:

If the car moves forward slowly as it speeds up, then its tires do not slip on the surface.

The rubber contacting the road moves towards the rear of the car, and static friction opposes relative sliding motion by exerting a force on the rubber towards the front of the car.

If the car is under control, the relative speed is zero along the lines where the rubber meets the road, and static friction acts rather than kinetic friction.

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