Unit 9: Application of Forces and Transfer of Energy

What is a Force? * A force is a push or a pull. * The SI unit for force is newton (N).

Contact Forces * When an object is touched, contact forces act on it (there is physical contact between 2 surfaces). - e.g. Friction, Elastic Force, Normal Force, Tension Force There are 5 contact forces: Friction Elastic Force Normal Force Resultant Force Applied Force

Non-Contact Forces * A non-contact force is a force which acts on an object without coming physically in contact with it. - e.g. Gravitational Force, Magnetic Force There are 2 non-contact forces: Gravitational Force Magnetic Force

Friction * force that opposes motion between 2 surfaces in contact * The mass of object and roughness of the surfaces in contact affects friction! - When the object is heavier/lighter and when surface is smoother/rougher, the amount of friction produced increases/decreases.

Applied Force * Force which is applied to an object by another object (literally). * acts in the opposite direction of elastic and tension force. - e.g. a man pushing a barrel; there is an applied force pushing the barrel

Elastic Force * force acting on a stretched or compressed elastic object to return to its original shape (e.g. springs) * acts in the opposite direction of applied force

Tension Force * Tension force is a contact force that occurs when an object is stretched. * acts in the opposite direction of applied force

Normal Force * Normal Force is defined as the force that a surface exerts on an object. * The surfaces of the objects must be in contact with each other (perpendicular) and they balance gravitational force/other forces.

Gravitational Force * Force that attracts two objects towards each other. * Gravity pulls things down. * Gravity pulls things radically inwards. * Note: Gravity is not the same as gravitational force! - Gravitational Force is the acting force between two objects. On the other hand, gravity is the force occurring between an object and the very big object (Earth). • Always use Gravitational Force (or weight) to answer qns! • e.g. When Earth exerts a gravitational force on an object and pulls it towards its centre, the object has weight. (refer to weight vs mass section)

Magnetic Force * Magnetic force is the push or pull between magnets only or magnets on magnetic materials. * like poles of magnets facing each other = attraction * unlike poles of magnets facing each other = repulsion * magnetic materials (Cobalt, Iron, Nickel and Steel) can attract magnets (but not repel them).

How to Measure Force? * Forces are measured with force-meters (e.g. spring balances). * The SI unit of force is the newton (N). * Equipment (spring balances) than can measure force (= N, weight) * There are two types of spring balances: - Compression spring balance (spring is compressed when a force acts on it.) - Extension spring balance (spring stretches when a pulling force acts on it)

Weight = mass x acceleration due to gravity W = mg * Note: g is a constant known as the acceleration due to gravity & is taken to be ≈ 10 m/s² • The weight of an object is the gravitational force acting on that object. • Since weight is a force, it is related to the equation • typically, 10N = 1kg = 1000g • only need to know how to calculate weight! • Weight is always linked to normal force (size of arrow drawn for diagrams must be same size in y3-y4)

• The transfer of energy between objects may cause changes in the size and/or shape of an object. • e.g. moulding plasticine • A force applied on an object can cause a turning effect about a fixed point called a pivot. • As the Force increases, the turning effect increases. • As the (perpendicular) Distance of the Force from the pivot increases, the turning effect increases. • Examples of turning effect of forces include using a fishing rod, opening a door etc. • (optional) *Formula for Turning Effect = Force x (perpendicular) distance from the pivot

• The distance between the pivot and the force (state source of force) is greater/lesser, hence lesser/greater force is needed to create the turning effect to (purpose).

• The distance between the pivot (source of force) and the weight of (item) is greater/lesser, hence there is greater/lesser turning effect to (purpose).

Pressure * Pressure is the force acting perpendicularly per unit area. * The SI unit for pressure: Pascal (Pa) * 1 Pa = 1 N/m² • Pressure = Force / Area Factors affecting Pressure * Force * Area • When force increases/decreases, pressure increases/decreases. • When area increases/decreases, pressure increases/decreases.

Factors affecting Pressure * Force * Area • When force increases/decreases, pressure increases/decreases. • When area increases/decreases, pressure increases/decreases.

• Mass = Amount of matter in an object (SI Unit = kg, remains constant regardless of location)

• Weight = Gravitational Force exerted on an object (SI Unit = N, changes depending on location) • Weight (N) = mass (m) x gravity (g, usually 10 m/s²) • Weight (N) can sometimes equal to Force (N) (e.g. when a book is falling)

• Work Done (J) = Force (N) x distance (d) • Work Done = the force multiplied by the distance moved in the direction of the force. • Conditions that must be present for work to be done: • There is a force acting on the object. • The object moves through a distance. • The movement of the object is in the direction of the force. • No work is done when: • Object does not move. • Direction of force is not in the same direction of the movement.

• Work Done (J) = Force (N) x distance (d)

• Conditions that must be present for work to be done: • There is a force acting on the object. • The object moves through a distance. • The movement of the object is in the direction of the force.

• No work is done when: • Object does not move. • Direction of force is not in the same direction of the movement.

Energy * Energy is the ability to do work. * Energy is transferred when work is done. * Energy is measured in Joules (J). * the amount of work done on an object is related to : 1. The energy transferred to the object 2. The force applied on the object 3. The distance travelled by the object