Forces
- A force is a push or a pull. Forces are measured in newtons (N) with a newton meter (force meter).
- Forces are drawn as arrows: the length shows the size and the arrowhead shows the direction.
- Contact forces need touching: friction, air resistance, water resistance, upthrust, tension and the reaction force from a surface.
- Non-contact forces act at a distance: gravity (weight), magnetism and static electricity.
Balanced and unbalanced forces
- Balanced forces are equal in size and opposite in direction. The resultant force is zero, so a still object stays still and a moving object keeps moving at the same speed.
- Unbalanced forces give a resultant force, which makes an object speed up, slow down or change direction.
- To find the resultant force, add forces acting in the same direction and subtract forces acting in opposite directions: 10 N to the right and 4 N to the left give 6 N to the right.
- Interaction pairs: when two objects interact, they push or pull on each other with equal and opposite forces. If you push on a wall, the wall pushes back on you.
Springs, friction and drag
- Extension = stretched length − original length. For a spring, the extension is proportional to the force (Hooke's law), up to a limit.
- Elastic deformation: the object returns to its original shape when the force is removed. Plastic deformation: it stays changed.
- Friction acts between surfaces and opposes motion. It is useful for grip and brakes, but wears parts and wastes energy as heat. Lubricants such as oil reduce it.
- Drag (air or water resistance) increases with speed. A streamlined shape reduces drag.
Accuracy and precision
- Accurate measurements are close to the true value.
- Precise measurements are close to each other when repeated.
Key terms
- Force
- A push or a pull, measured in newtons (N).
- Newton meter
- An instrument that measures force.
- Resultant force
- The single force that has the same effect as all the forces acting on an object.
- Balanced forces
- Equal and opposite forces, so the resultant force is zero.
- Friction
- A force between surfaces that opposes motion.
- Air resistance
- A force that opposes objects moving through air.
- Upthrust
- The upward push of a liquid or gas on an object.
- Weight
- The force of gravity on an object.
- Interaction pair
- Two equal and opposite forces that two objects exert on each other.
- Extension
- How much longer a spring gets when it is stretched.
- Elastic deformation
- A change of shape that goes back when the force is removed.
- Streamlined
- Shaped to reduce air or water resistance.