Edexcel IGCSE Physics: Double Science 复习笔记： 1.2.3 Resultant Forces

• 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

• 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

Diagram showing the resultant forces on three different objects

• 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

   

Step 1: Add up all of the forces directed to the right

4 N + 8 N = 12 N

Step 2: Subtract the forces on the right from the forces on the left

14 N – 12 N = 2 N

Step 3: Evaluate the direction of the resultant force

• • The force to the left is greater than the force to the right therefore the resultant force is directed to the left

   

Step 4: State the magnitude and direction of the resultant force

• • The resultant force is 2 N to the left

   

• Friction is defined as:

The force which opposes the motion of an object

• Frictional forces always act in the opposite direction to the object's motion
• Friction emerges when two (or more) surfaces rub against each other:
  • At a molecular level, both surfaces contain imperfections - i.e. they are not perfectly smooth
  • These imperfections tend to push against each other

   

• The image below show the force of friction acting on a downhill sledder:

Friction is a force which opposes an objects motion, acting in the opposite direction to it