Forces - Resultant Forces

Recognizing the forces acting on an object is the key to working out the resultant force.

If you find the resultant of several forces is not zero, then there will be an acceleration in the direction of the resultant force. If the object is already moving, the effect of this acceleration can make the object speed up, slow down or change direction. Where a resultant force opposes the motion, the acceleration is negative (a deceleration) and the object slows down. Where the force is in the same direction as the motion, the acceleration is positive and the object speeds up. If the resultant force is at an angle to the direction of motion, the object changes direction.

The senior high school examiners like to use a car as an example of resultant forces but they could use anything. For a car, the forces of interest to the motion are the driving force, air resistance and friction. Let's start with the car at rest. Air resistance and friction are zero, the driving force is zero so the resultant force is zero. The driver then starts the engine and engages the gears. The engine provides a driving force. The driving force is now larger than the air resistance and friction therefore the resultant force is no longer zero and the car accelerates.

But as it does so, the air resistance and friction both increase and so the resultant force decreases. The acceleration of the car becomes less and when the resultant force is zero again, the car moves at a constant velocity. When the driver turns the steering wheel, that introduces a sideways force into the system. The resultant force is no longer zero. Because the new force is at an angle to the other forces, the resultant force is also at an angle, therefore the car changes direction. When the driver removes this sideways force by straightening the steering wheel, the resultant force drops back to zero in all directions and the car continues on, in the new direction.

One of the things that you may be asked to do in the exam is to predict what happens when resultant forces change magnitude. Firstly, you need to consider in what direction each force is acting. Next you should consider the initial state of the object in the question - was it already moving or stationary? Finally, you can perform your calculation to work out the magnitude and direction of the new resultant force. Once you have done all that, you should be able to predict the effect. Don't be afraid to use diagrams to help you.