Dynamics
Balanced and unbalanced forces, action–reaction pairs, one-dimensional free-body diagrams, F = ma and friction.
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Before you begin
Dynamics explains why motion changes: a resultant force makes a body speed up, slow down or change direction. The lessons start by drawing the forces on one body, along one line, in a free-body diagram, combine those forces into a resultant, and use F = ma to find the acceleration it causes. They finish with friction and with the action–reaction pairs that act between two interacting bodies.
Be comfortable with: kinematics, because acceleration is what a resultant force produces, and forces, mass and weight.
Learning goals
- Draw free-body diagrams for one-dimensional force systems
- Describe the effect of balanced and unbalanced forces on a body
- Describe ways a force may change motion
- Identify action–reaction pairs on interacting bodies
- Apply resultant force = mass × acceleration
- Explain the effects of friction on motion
Syllabus statements covered
- apply Newton's Laws to: (i) describe the effect of balanced and unbalanced forces on a body (ii) describe the ways in which a force may change the motion of a body (iii) identify action-reaction pairs acting on two interacting bodies (stating of Newton's Laws is not required)
- identify forces acting on a body and draw free body diagram(s) representing the forces acting on the body (for cases involving forces acting in one dimension)
- recall and apply the relationship resultant force = mass × acceleration to new situations or to solve related problems
- explain the effects of friction on the motion of a body
Lessons
Work through them in order.
- Free-body diagramsChoose one body, identify every force acting on it and draw its free-body diagram.
- Resultant force and balanced motionCombine the forces on one body and decide from the resultant whether and how its motion changes.
- Newton's second law: F = maFind the resultant force first, then use F = ma to calculate acceleration, mass or an unknown force with its direction.
- Friction and its effects on motionWork out the direction of friction or air resistance, include it in the resultant force and explain its effect on motion.
- Action–reaction pairsIdentify the action–reaction pair in an interaction and tell it apart from balanced forces on one body.
Practise and check
Topic reference
Quick reference
| Idea | Relationship or rule | Note |
|---|---|---|
| balanced forces | resultant force is zero | at rest, or moving at constant velocity |
| unbalanced forces | resultant force is not zero | the body accelerates in the resultant’s direction |
| resultant force | resultant force = mass × acceleration, F = ma | F in N, m in kg, a in m/s² |
| free-body diagram | every force acting on one body | forces along one line |
| action–reaction pair | equal in size, opposite in direction | act on two different bodies |
| friction | opposes sliding, or the tendency to slide | also heats the surfaces |
Core knowledge to remember
- Balanced forces on a body leave its motion unchanged; unbalanced forces change its speed, its direction or both.
- A force may change the motion of a body by making it start, stop, speed up, slow down or change direction.
- Action–reaction pairs act on two interacting bodies.
- Resultant force = mass × acceleration.
- Friction opposes the relative motion of surfaces in contact and can slow a body down.
Common mistakes
- Using one force from the diagram in F = ma instead of the resultant force.
- Saying a moving body must have a resultant force on it. At constant velocity the resultant is zero.
- Drawing forces the body exerts on other things on its free-body diagram.
- Saying action–reaction forces cancel. They act on different bodies.
- Saying friction always stops motion. It opposes sliding, and without it you could not walk or drive.