Resultant Forces (Cambridge (CIE) IGCSE Physics): Revision Note

Effects of forces

• A force is defined as:

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

• Forces can have a variety of effects on an object

• Forces can change an object's

  • speed

  • direction

  • shape

  • size

Effects of forces on different objects

A thrust force can cause a car to speed up, a gravitational force can cause a comet to change direction, compression forces can cause a spring to change shape

• The effects of forces on an object often depend on the type of force acting

  • The push force (thrust) of an engine can cause a car to speed up, whilst the force exerted by the brakes (friction) can cause it to slow down

  • The gravitational pull of the Sun on a comet causes the comet to change direction

  • When two opposing forces push on each end of a spring, the spring changes shape (it compresses)

Resultant forces on a straight line

What is a resultant force?

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

• When multiple forces act on one object, the forces can be combined to produce one net force that 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 net force experienced by the object

Balanced and unbalanced forces

• The forces acting on an object can be described as balanced or unbalanced

• Forces are balanced if multiple forces act in opposing directions with an equal magnitude in each direction

  • The effects of the forces then cancel out

  • There is no resultant force in that plane of direction

• Forces are unbalanced if the effects of the forces acting in each plane do not cancel out

  • There is a resultant force in one or more planes of direction

• A book is at rest on a table

  • The gravitational pull of the Earth on the book (weight) acts in a downward direction

  • The push force of the table on the book (normal contact force) acts in the upward direction

  • The forces are equal in magnitude and opposite in direction

  • The forces are therefore balanced

  • There is no resultant force acting on the book

Zero resultant force on a book resting on a table

A book resting on a table is an example of balanced forces

Calculating resultant force

• Force is a vector quantity; it has both magnitude and direction

• When adding forces together, it is important to assign positive and negative values to show the direction in which the forces are acting

• If a 5 N force acts to the right and a 5 N force acts to the left on an object, then we assign one of the values as positive and one as negative

• So the resultant force acting on the object is

• The forces acting on the object are equal in magnitude and opposite in direction; therefore, they cancel one another out

• This is like two people pushing a box with equal force from opposite sides; the box doesn't move

• If two people push the box from the same side in the same direction, one with a 3 N force and one with a 7 N force, then the forces will add together, and the box will move in the direction of the resultant force

• If two people push the box in opposite directions, one with a 7 N force to the left (negative) and one with a 3 N force to the right (positive), then the forces will add together and the box will move in the direction of the resultant force

• In this case, the box will move to the left with a force of 4 N

Zero and non-zero resultant forces

Zero and non-zero resultant forces acting on three objects