Motion and Its Causes

Force and Newton’s Laws of Motion

What happens if more than one force acts on an object?

You can explore this question with your toy car or ball. Try exerting two forces, like two finger taps in the same direction. You can see that the car or ball moves faster. The forces add. What happens if the two forces are in opposite directions? You can try pushing each end of a motionless toy car. What happens if both forces are equal? If one is stronger than the other? If they’re both equal, the car will remain motionless. That is, it will act as if there is no force on it. If one is stronger than the other, then it will move in the direction of the smaller force, but it will accelerate less. That is, the results will be the difference between the two forces. We’ll explore forces that act in different directions later.

Physicists say that the combination, addition, or subtraction of forces produces a net force and it is the net force that affects the acceleration. Thus Newton’s laws should be written as follows.

Newton’s First Law: If there is no net force on an object, then if it was at rest it will remain at rest. If it was moving, it will continue to move at the same speed and in the same direction.

Newton’s Second Law: If a net force acts on an object, it will accelerate in the direction of the force. The acceleration will be directly proportional to the net force and inversely proportional to the mass.

That is, a = Fnet/m. Or, Fnet = ma.

