From the post on acceleration we can deduce that there are four changes that can take place if a force acts on an objects:
- The speed of the object will increase from zero if it was at rest and the object will move in the direction in which the force is acting.
- The speed of the object will increase if the force is acting in the same direction as the the direction of motion of the object until the force no longer acts on the object.
- The speed of the object will decrease if the force is acting in a direction opposite to the direction of motion of the object. The speed of the object might decrease to zero if the force acts for long enough.
- If the object is moving and the force is acting in a direction perpendicular to the direction of motion of the object then the object will move in a circular path. When the application of the force stops the object will leave the circular path and would move in a straight line in a direction tangential to the circular path.
These four changes can be combined together to produce the Newton’s first law of motion as shown below:
Unless a force is applied an object will continue in its state of rest or uniform motion in a straight line.
Another way to write this law:
If a force is applied (i) an objects that was at rest will no longer be at rest or (ii)if moving with a uniform will no longer have a uniform or constant speed or lastly (iii) if moving in a straight line will no longer move in the same direction
This law is thus explaining that if a force is acting on an object then the object will experience an acceleration. However we are officially learn this only when we see the Newton’s second law of motion.