Resultant Forces (Cambridge (CIE) O Level Physics): Revision Note

Types of Force

• A force is defined as:

  A push or a pull that acts on an object due to the interaction with another object

• Forces can affect bodies in a variety of ways:

  • Changes in speed: forces can cause bodies to speed up or slow down

  • Changes in direction: forces can cause bodies to change their direction of travel

  • Changes in shape: forces can cause bodies to stretch, compress, or deform

Different forces

The effects of different forces on objects

Tension

• Tension is:

  The force experienced by a cable, rope, or string when pulled, hung, rotated or supported

• This is normally labelled as T on free body diagrams

Tension force on a Picture Frame

Tension always acts away from the mass

Normal Contact Force

• The normal contact force is:

  The force arising when an object rests against another object acting at a 90° angle to the plane of contact

• It is sometimes also referred to as the reaction force

• This is normally labelled as N or R on free body diagrams

• This force arises from Newton's Third Law

Normal Contact Force

Normal contact force is the force with which a surface pushes against an object. The normal contact force always acts perpendicularly to the surface

Upthrust

• Upthrust is:

  The upward buoyancy force acting on an object when it is in a fluid

• Upthrust can occur in liquids and gases

Upthrust on a Boat

Upthrust always acts upwards

Friction

• Friction is:

  The force that arises when two surfaces are in contact with each other

• Friction always opposes the motion

• This is normally labelled as F or Fr on free body diagrams

Force of Friction Acting on a Car

Friction always acts at the point where the objects are in contact, and in the opposite direction to the direction of motion

Identifying Forces Acting on an Object

• Free body diagrams are useful for modelling the forces that are acting on an object

• Each force is represented as a vector arrow, where each arrow:

  • Is scaled to the magnitude of the force it represents

  • Points in the direction that the force acts

  • Is labelled with the name of the force it represents

• Free body diagrams can be used:

  • To identify which forces act in which plane

  • To resolve the net force in a particular direction

Force Diagrams of a Mass on a Spring, and a Parachuter

Free body diagrams can be used to show the various forces acting on objects

Resultant Forces on a Straight Line

• A resultant force is a single force that describes all of the forces operating on a body

• When many forces are applied to an object they can be combined (added) to produce one final force which describes the combined action of all of the forces

• This single resultant force determines:

  • The direction in which the object will move as a result of all of the forces

  • The magnitude of the final force experienced by the object

• The resultant force is sometimes called the net force

• Forces can combine to produce

  • Balanced forces

  • Unbalanced forces

• Balanced forces mean that the forces have combined in such a way that they cancel each other out and no resultant force acts on the body

  • For example, the weight of a book on a desk is balanced by the normal force of the desk

  • As a result, no resultant force is experienced by the book, the book and the table are equal and balanced

Balanced Forces Acting on a Book at Rest on a Table

The upward acting normal contact force is equal to the force of weight, therefore the forces acting on the book are balanced

• Unbalanced forces mean that the forces have combined in such a way that they do not cancel out completely and there is a resultant force on the object

  • For example, imagine two people playing a game of tug-of-war, working against each other on opposite sides of the rope

  • If person A pulls with 80 N to the left and person B pulls with 100 N to the right, these forces do not cancel each other out completely

  • Since person B pulled with more force than person A the forces will be unbalanced and the rope will experience a resultant force of 20 N to the right

Unbalanced Forces in a Game of Tug-of-War

The force exerted on the rope by Person B is greater than the force exerted on the rope by Person A, therefore the forces acting on the rope are unbalanced

• Resultant forces can be calculated by adding or subtracting all of the forces acting on the object

  • Forces working in opposite directions are subtracted from each other

  • Forces working in the same direction are added together

• If the forces acting in opposite directions are equal in size, then there will be no resultant force – the forces are said to be balanced

Resultant Forces on Different Objects

The resultant force is the net force acting on the object when all the individual forces have been added and the directional components accounted for

• Imagine the forces on the boxes as two people pushing on either side

  • In the first scenario, the two people are evenly matched - the box doesn't move

  • In the second scenario, the two people are pushing on the same side of the box, it moves to the right with their combined strength

  • In the third scenario, the two people are pushing against each other and are not evenly matched, so there is a resultant force to the left