1. Introduction

It is an everyday experience that if you push or pull a stationary object hard enough, it will start to move.

It is also our experience that it is more difficult to start larger objects (like railway carriages) moving than small ones (like tennis balls).

It is also more difficult to get an object to move fast (or to stop it when it is moving fast) than to get the same object to move slowly (or to stop it when it is moving slowly).

In the late sixteenth century, Galileo Galilei began formulating rules to describe these experiences more precisely. Sir Isaac Newton later formulated his own observations and insights as well as those of his predecessors into three LAWS OF MOTION. These laws describe how bodies respond to forces and are the postulates () of NEWTONIAN DYNAMICS. They are only justified because they lead to results which are consistent with our observations.

2. Newton's first law of motion

Newton's first law defines the idea of a force. It states the fact that when a resultant force acts on a body, its state of motion will change. What it means is that when an object is subjected to a net force (), it will either begin to move (if the object was initially at rest), or begin to move faster (accelerate), or slow down (decelerate). The object may also change its direction of motion.

Newton's First Law of Motion
"A body will continue in a state of rest or uniform motion in a straight line unless acted upon by a resultant force."

The tendency of a body to remain at rest or to continue moving in a straight line is called its INERTIA.

A lighter moment: what happened here?

3. Friction

It is a common experience that objects do not continue moving indefinitely once they are set in motion. We also observe that in order to get a stationary object to move, a minimum force must be applied. This is because of the FORCE OF FRICTION.

Friction is a CONTACT FORCE. It acts between any two surfaces IN CONTACT () in a direction TANGENTIAL and OPPOSITE to the direction of motion:

In order to start an object moving it is necessary for the applied force to exceed the force of friction. Thus, looking at the above diagram, if the applied force is F, and the frictional force is f, the object will move only when F > f.

There are three basic types of friction:

3.1 Static friction

The diagram on the left shown an object lying on a surface inclined at an angle θ to the horizontal. If θ is relatively small, the object will remain at rest, because the frictional force is greater than the vertical component of the weight of the object. However, one can increase θ to a point where the object starts to slide down.

3.2 Kinetic friction

This is the friction that operates in such a way as to oppose the relative motion of surfaces that are sliding past one another. In the diagram above, kinetic friction operates when the body is in motion down the inclined plane. A vehicle moving at a steady speed along a road is subject to kinetic friction between the tires and the road surface. The moving parts of the engine are also subject to kinetic friction.

3.3 Drag from fluids

If a body moves through a fluid (such as air or water), the fluid exerts a frictional force on the moving object. The magnitude of this drag depends on a number of factors, such as the velocity of the object through the fluid and the area presented by the moving object perpendicular to the direction of motion.

Bodies falling through air accelerate, and the increasing speed causes an increased drag. A point is eventually reached where the drag equals the weight of the object, and the body falls at a constant speed, called the TERMINAL SPEED. For a sky-diver in free fall, this is about 200 km·hr-1. A parachute is basically a device that is designed to increase drag, and when it is open, it greatly lower the terminal speed, allowing the sky-diver to land safely.

Modern vehicles are designed to reduce drag, which has a negative influence on fuel consumption, by STREAMLINING their shapes. Consider the shape of the two cars shown below. For a given speed, the one on the left will experience greater drag forces than the one on the right.

Additional questions

Newton's three laws of motion

First law: A body will continue in a state of rest or uniform motion in a straight line unless acted upon by a resultant force.

Second law: The rate of change of momentum of a body is proportional to the force applied to that body and in the direction of that force.


When a resultant force is applied to a body it produces an acceleration that is directly proportional to the resultant force and inversely proportional to the mass of the body).

Third law: For every force (or action) between two bodies there is always an equal but oppositely directed force (or reaction).


Someone clearly had never heard of Newton's first law!