If only external forces can cause the center of mass of a system of particles to accelerate, how can a car on level ground ever accelerate? We normally think of the car’s engine as supplying the force needed to accelerate the car, but is this true? Where does the external force that accelerates the car come from?
Try to accelerate that car on smooth ice with bald tires.
To understand where the external force comes from that accelerates a car on level ground, we need to consider the interaction between the car and its surroundings. The engine of a car indeed supplies the force required to accelerate the car, but it might not be the ultimate source of the external force.
One fundamental principle to remember is Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. When the car's engine exerts a force on the wheels (action), the wheels, in turn, exert an equal and opposite force on the ground (reaction).
The external force that accelerates the car originates from the reaction force exerted by the wheels on the ground. As the wheels push backward against the ground, the ground pushes forward on the wheels. This interaction creates a net external force on the car, causing it to accelerate.
It's important to note that the force from the wheels against the ground might not provide an immediate acceleration. Due to factors like inertia and friction, there might be a delay before the car starts moving. However, once the forces overcome these factors, the car begins to accelerate.
In summary, while the car's engine supplies the force required to propel the car, the external force that ultimately accelerates the car comes from the reaction force exerted by the wheels on the ground. It is this interaction between the car and its surroundings that allows the car to accelerate on level ground.