Newton's First Law

Summary

A force is a push or a pull and can include contact or non-contact forces

  • A contact force is exerted where two bodies touch each other, and a non-contact force is when a force is exerted over a larger distance (e.g. gravitational, electric and magnetic fields)

The strength of a gravitational field is measured in N kg-1

  • On Earth, there is a gravitational field strength of 9.8 N kg-1

Weight = mass x gravitational field strength

Newton's first law of motion: a body will remain at rest or continue to move in a straight line with a constant velocity unless it is acted on by an unbalanced force

Newton's second law of motion: when an unbalanced (or resultant) force acts on a body, it will accelerate in the direction of that force. The size of the acceleration may be determined by using the equation F = ma

Newton third law of motion: when body A exerts a force, F, on body B, body B exerts an equal and opposite force, F, on body A

Introduction

A body may be subject to many different forces, and that body may exert forces on other bodies

To understand the effect of forces on a body, we draw a free body diagram, which shows all the forces acting on a single body; no other body is shown in the diagram

Free Body Diagram

Example

Free body diagram

Here the parachute and skydiver fall at a constant speed

The free body diagram here is easy as the skydiver and parachute aren't in contact with anything, except the air

Simply, then, the drag (D) balances the weight (W) of the skydiver and the parachute

Example 2

Free body diagram

Here a rock climber is abseiling and pauses for a rest - to draw a free body diagram there must only be one body, so we must remove the rock

The three forces are: the climber's weight (W), the rope's tension (T), and reaction force from the rock (R)

These forces add up to 0 as the climber is stationary

Example 3

Free body diagram

The forces acting on the ladder are:

  • RW, a horizontal reaction force from the wall
  • RF, a vertical reaction force from the floor
  • F, a horizntaol frictional force from the floor
  • W, the weight of the ladder
  • RM, a contact force from the man that is equal in size to his weight (this is not the man's weight, which acts on him)

Since the ladder remains stationar, the forces on it balance

  • So RF = W + RM (these are the forces acting vertically)
  • So F = RW (these are the forces acting horizontally)