Sir Isaac Newton discovered three laws of motion. Newton’s first law states that “Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.” (Newton's Three Laws of Motion) If an object is not moving, it will not move unless a force moves it, and an object in motion will not accelerate or decelerate unless a force accelerates or decelerates it. Newton’s second law states that force = mass * acceleration “Acceleration and force are vectors … in this law the direction of the force vector is the same as the direction of the acceleration vector.” (Newton's Three Laws of Motion)The force that changes an objects state of motion is the product of the objects mass and the amount that the object accelerates. Newton’s third law states that “For every action there is an equal and opposite reaction.” (Newton's Three Laws of Motion) For every force applied there is an equal and opposite force.
Energy is conserved. The spring in the mousetrap has potential energy because the spring is pulled back and retains that energy. The car will not move unless the spring is released. (Newton’s First Law) When the spring is released, the potential energy is converted into Centripetal force, moving the arm. The arm acts as a lever, and also has Centripetal force. The arm pulls the string, giving it tension. The string turns the axle and the wheels, giving it Centripetal force. The wheels contact the ground and because friction works in the opposite direction as the car, the car is able to move forward (Newton’s Third Law). Friction is required. Without friction, the car would not be able to move forward.
According to Newton’s First Law, the car would remain in motion for ever, as long as external forces are not applied. The car eventually stops, because of the friction that was required to start to move the car. The spring has to have enough force bring the car the three meters, after overcoming inertia. Friction is the applied force vector for the car, to the car will accelerate in the direction of the friction. (Newton's Second Law) Friction is the equal and opposite force from Newton's Third Law, so the direction of the friction vector is opposite of the direction of the car's vector. The car decelerates because the friction's opposite direction causes negative acceleration.