Forces which are being applied to objects are usually described as "resultant forces". It is the size (or magnitude) of the force together with the direction in which the force is acting which will determine what happens to the body/object concerned.




Here we see a blue ball. Is it moving? Is it stationary? Why isn't it moving (if it isn't)? Why is it moving (if it is)?


There are lots of questions we can ask about this simple object, and we can often answer them when we consider the unseen, in other words the resultant forces that may or may not be acting on the object. Let's take another look at the ball, but this time with what we can't ordinarily see depicted with arrows.


This time we can see that the ball is probably not moving (and I will explain why I say "probably" shortly) because any tendency to move up is being counteracted by the tendency to move down, similarly any tendency to move the left is being counteracted by a tendency to move to the right the "result" (and this is where we get resultant from) is that the ball "probably" stays still.


Or probably not ?


Consider the same ball floating through space at a constant velocity. It is sufficiently far away from any planet, star, asteroid, comet what if you want to not be influenced by any gravitational field. Is it accelerating? Well, no because we have said it is moving at a constant velocity, similarly it is not slowing down so we can say that the "left" and "right" forces are cancelling each other it, i.e. there is no "resultant" force pushing the ball forward. Similarly there is no attraction due to gravity (for the reasons that we have explained) and therefore no need for opposite force to counteract it.



Newton's first law states that a resultant force is needed to make something start moving, accelerate, decelerate or change direction. So if we go back to our ball we can say:


1. If the resultant force on a stationary object is zero the object will remain stationary.


2. If the resultant force on a moving object is zero it will just carry on moving at the same velocity.


This can sometimes be a difficult concept for students to grasp, it seems obvious that something that isn't moving won't just move on its own, but students think that if an object is moving there must be something pushing it along, if something was pushing the object it would start to accelerate (or decelerate, depending on which direction the "pushing" force was coming from).


Consequently,

  • An object that is at rest will stay at rest unless a force acts upon it.
  • An object that is in motion will not change its velocity unless a force acts upon it.


This is known as uniform motion. An object continues to do whatever it happens to be doing unless a force is exerted upon it. If it is at rest, it continues in a state of rest (demonstrated when a tablecloth is skilfully whipped from under dishes on a tabletop and the dishes remain in their initial state of rest). If an object is moving, it continues to move without turning or changing its speed. This is evident in space probes that continuously move in outer space. Changes in motion must be imposed against the tendency of an object to retain its state of motion. In the absence of net forces, a moving object tends to move along a straight line path indefinitely.


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