Chapter 5 Notes: Exploring Forces

A force is a push or pull applied on an object resulting from its interaction with another object. At least two objects must interact for a force to come into play. Forces can:

• Make a resting object move
• Change the speed of a moving object
• Change the direction of motion
• Change the shape of an object

Contact forces act only when two objects are physically touching.

Muscular Force: Force produced by the action of muscles. Used by humans and animals for walking, running, lifting, etc.

Friction: The force that acts between two surfaces in contact when an object moves or tries to move. It always acts opposite to the direction of motion. Friction arises due to tiny irregularities on surfaces — rougher surfaces have more friction.

Non-contact forces act even without physical contact between objects.

Magnetic Force: Force exerted by a magnet on another magnet or magnetic material. Like poles repel; unlike poles attract.

Electrostatic Force: When certain materials are rubbed together, static charges build up. A charged object can attract uncharged objects. Like charges repel; unlike charges attract.

Gravitational Force: The force with which the Earth pulls every object towards itself. It is always attractive and acts on all objects.

Weight is the force with which the Earth pulls an object towards itself. It is measured in Newtons (N).

Mass is the amount of matter in an object, measured in grams (g) or kilograms (kg). Mass remains the same everywhere.

Weight can change from place to place (e.g., on the Moon, weight is 1/6th of weight on Earth) but mass never changes.

A spring balance measures weight — when an object is hung from its hook, the spring stretches and the scale shows the weight in Newtons.

When an object is placed in a liquid, two forces act on it:

• Gravitational force — pulls the object downward
• Buoyant force (Upthrust) — liquid pushes the object upward

If gravity > buoyant force → object sinks If buoyant force = gravity → object floats

Archimedes' Principle: An object immersed in a liquid experiences an upward force equal to the weight of the liquid it displaces.