Newton’s First Law of Motion

Newton’s First Law of Motion, as well as the other laws of motion, were formulated by physicist, mathematician and astronomer who studied universal gravitation Sir Isaac Newton (1643-1727). See Newton’s Second Law and Newton’s Third Law of motion.

Forces

When the motion of a body changes, such as in increase or decrease in velocity for example, the body is being acted upon by one or more forces.ย Forces include weight, friction, normal reaction, tension, thrust, compression, resistance, etc. Forces are measured in Newtons (N) which is equivalent to 1 kilogram metre per second squared (kg m/s/s).

Newton’s First Law of Motion

When the motion of a body changes, such as an increase or decrease in velocity for example, the body is being acted upon by one or more forces. Forces can also be acting on a stationary object. If forces are acting on a body and it doesn’t move, the forces balance and we say the body is in equilibrium.

Newton’s First Law states that: 

  1. A stationary body will remain at rest unless a force acts upon it.
  2. A moving body will have constant velocity unless a force acts upon it.

In both of these cases, the acceleration is constant at zero.

The second point can be a little harder to imagine. So, consider astronauts or objects in space. Click here to see some astronauts at a space station https://www.youtube.com/watch?v=lreFwShgT1Y. Once they begin to move, they seem to continue to move effortlessly. This is because there are fewer forces working in space. Once the force that moves the object has been applied, objects are moving with constant velocity.

We can consider the forces acting on a body in any given direction. If the body is not moving in that direction then the resultant force ( = forces in given direction – forces in opposite direction) must be zero. The same can be said for an object that is moving with constant velocity i.e. not accelerating. Mathematically, this can be written as

\sum_i {\bf F}_i = {\bf a}={\bf 0}

Note that {\bf F}_i is the ith force where bold represents vectors and so this statement represents the fact that resultant force is zero in all directions.

Examples

A conker of weight 30 grams hangs in equilibrium at the bottom of a light inextensible string. Find the tension in the string.

Solution:

The resultant force on the conker must be zero. This means that the force due to gravity must match the tension in the string. The force due to gravity is weight which is mass times the gravitational constant which we take to be g=9.8. Hence, the tension and the gravitational force are both 0.03 \times 9.8= 0.294 Newtons, where 30 grams has been converted to kilograms.

A meteorite travels through outer space at a constant velocity. Identify the forces acting on the meteorite.

Solution:

This is a trick question, there are no forces acting on the meteorite.

Videos

https://youtu.be/RrF8d0bYXjc

This video gives a very basic introduction to balanced and unbalanced forces and Newton’s First Law of Motion.

AS Mechanics Forces & Newton’s Laws

AS Mechanics Forces & Newton’s Laws