Study Notes
Forces can change the motion, size, or shape of an object. They can be described by Newton's laws and can be represented by resultant forces, which determine the overall effect on an object.
- Resultant Force — The net force acting on an object, determining its motion. Example: If two people push a box from opposite sides with equal force, the resultant force is zero, and the box doesn't move.
- Newton's First Law — An object remains at rest or in uniform motion unless acted upon by an external force. Example: A book on a table stays still until someone pushes it.
- Newton's Second Law — The acceleration of an object is proportional to the net force acting on it and inversely proportional to its mass (F = ma). Example: A car accelerates faster when a greater force is applied to it.
- Newton's Third Law — For every action, there is an equal and opposite reaction. Example: When you jump off a boat, the boat moves backward.
- Friction — The force resisting the relative motion of surfaces sliding against each other. Example: Rubbing your hands together generates heat due to friction.
- Hooke's Law — The extension of a spring is directly proportional to the force applied to it (F = kx). Example: Stretching a spring with a weight and measuring its extension.
- Moment of a Force — The turning effect of a force about a pivot point. Example: Using a wrench to turn a bolt.
Exam Tips
Key Definitions to Remember
- Resultant Force
- Newton's Laws of Motion
- Friction
- Hooke's Law
- Moment of a Force
Common Confusions
- Mixing up balanced and unbalanced forces
- Misunderstanding the direction of friction
- Confusing mass with weight
Typical Exam Questions
- What is the resultant force on an object if two forces of 5 N and 3 N act in opposite directions? The resultant force is 2 N in the direction of the larger force.
- How does Newton's third law apply when you push against a wall? The wall pushes back with an equal and opposite force.
- What happens to the extension of a spring if the force applied is doubled? The extension doubles, assuming the limit of proportionality is not exceeded.
What Examiners Usually Test
- Understanding and application of Newton's laws
- Calculating resultant forces and moments
- Interpreting extension-load graphs
- Describing the effects of friction and air resistance