Forces

Study Notes

Forces can change the motion, size, or shape of an object. They can be balanced or unbalanced, affecting how an object moves or stays still.

• Resultant Force — the net force acting on a body, determining its motion direction.
  Example: A car accelerating when the engine force exceeds friction.
• Newton's First Law — an object remains at rest or in uniform motion unless acted upon by an external force.
  Example: A book resting on a table stays still until pushed.
• Newton's Second Law — the force on an object is equal to its mass times its acceleration (F = ma).
  Example: A 2 kg ball accelerating at 3 m/s² experiences a force of 6 N.
• Newton's Third Law — every action has an equal and opposite reaction.
  Example: A swimmer pushes water backward, propelling forward.
• Friction — the resistance force between two surfaces in contact, impeding motion.
  Example: Sliding a book across a table.
• Hooke's Law — the extension of a spring is directly proportional to the force applied, within the elastic limit (F = kx).
  Example: Stretching a spring with weights.
• Moment of a Force — the turning effect of a force about a pivot, calculated as force times perpendicular distance from the pivot.
  Example: Opening a door by pushing at its edge.

Exam Tips

Key Definitions to Remember

• Resultant Force: The net force acting on an object.
• Newton's Laws: Three fundamental principles describing motion.
• Friction: Resistance to motion between surfaces.
• Hooke's Law: Relationship between force and extension in springs.
• Moment of a Force: Turning effect of a force about a pivot.

Common Confusions

• Confusing balanced and unbalanced forces.
• Misunderstanding the direction of friction.
• Mixing up mass and weight.

Typical Exam Questions

• What is the resultant force on a stationary object?
  Zero, if no external force acts.
• How does friction affect motion?
  It resists motion, causing objects to slow down or stop.
• What happens to an object in equilibrium?
  It remains at rest or moves at constant velocity.

What Examiners Usually Test

• Application of Newton's laws to real-world scenarios.
• Calculating resultant forces and moments.
• Understanding and interpreting extension-load graphs.