Motion and Its Causes
Force and Newton’s Laws of Motion
What are some applications of the Third Law?
How do you accelerate your car? You press on a pedal called the accelerator. Does that cause the car to speed up? It cannot because the net force that causes the acceleration must be exerted on the car from outside it.
What does the car interact with? When it is not moving, it is touching the road, and thus interacting with it. When you press on the accelerator the engine causes the wheels to rotate in a direction that, because of friction between the tires and the road, pushes backward on the road. By Newton’s Third Law, then, the road pushes forward on the tires and the car accelerates in its forward direction. What happens if you car is on ice? Often then the friction between the tires and the ice is so small that the wheels can’s exert enough backward force on the ice for the ice to exert the force needed to accelerate the car.
Note that friction, instead of being a bad thing, as suggested earlier, is needed in this case. How else is friction useful in accelerating things? How do you walk or run? Your feet are also interacting with the ground. As long as there is sufficient friction, when you push your feet backwards, the ground pushes you forward.