Force (Grade 11 NSC Matric Physical Sciences): Revision Notes

Force

What is force?

Force is anything that can cause a change to objects. Forces are incredibly important in physics because they explain how and why things move, stop, or change shape around us.

Force is a vector quantity, which means it has both magnitude (size) and direction. We use the symbol $\vec{F}$ for force, and the SI unit for force is the newton, with symbol N. This unit is named after Sir Isaac Newton, who first defined the concept of force scientifically.

Contact and non-contact forces

Forces can be classified as either contact forces or non-contact forces.

Contact forces

A contact force must touch or be in contact with an object to cause a change. When you push a door open, your hand makes contact with the door - this is a contact force in action.

Examples of contact forces include:

• The force used to push or pull things, like opening or closing a door
• The force a sculptor uses to turn clay into a pot
• The force of wind turning a windmill

Non-contact forces

A non-contact force does not have to touch an object to cause a change. These forces can act across distances without any physical contact.

Examples of non-contact forces include:

• Gravity - like the Earth pulling the Moon towards itself
• Electricity - like a proton and electron attracting each other
• Magnetism - like a magnet pulling a paper clip towards itself

Resultant force

When working with forces, we often need to consider the resultant force acting on an object. The resultant force is simply the vector sum of all the forces acting on the object.

Types of forces in physics

Physics helps us understand the natural world, and you probably know more physics than you think! You see forces in action every day - when you throw a stone, it eventually falls to the ground due to gravity.

Let's explore some important types of forces by considering a practical situation. Imagine you have a table and some books with different masses. When you place a book on a flat table, nothing happens - the book just rests there. But if you slowly lift one side of the table so the top becomes tilted, the book will eventually start to slide off. This simple situation demonstrates several important forces.

Normal force

When an object is placed on a surface, several forces come into play. If you put a book on a table, gravity pulls the book downward. However, the table stops the book from falling to the floor. The table must exert an upward force on the book to balance out the downward gravitational force.

The normal force is what prevents objects from falling through surfaces. When you stand on the ground, the gravitational force pulls you down, but the ground exerts an equal and opposite normal force upward to balance this out.

Friction forces

Why does a box sliding on a surface eventually come to a stop? The answer is friction. Friction arises when two surfaces are in contact and moving relative to each other.

You can experience friction by pressing your hands together and moving one backwards and forwards. Your hands get warm because friction generates heat through the interaction of the surfaces.

Friction occurs because surfaces interact with each other. Even smooth-looking surfaces have tiny bumps and grooves. When surfaces slide against each other, these microscopic irregularities catch and resist the motion.

Static and kinetic friction

There are two main types of friction:

Static friction occurs when an object is not moving. If you try to push a heavy crate but it doesn't move, static friction is balancing your applied force. Static friction can vary from zero up to a maximum value, depending on how hard you push.

Kinetic friction occurs when an object is already moving. Once the crate starts sliding, kinetic friction acts to slow it down. Kinetic friction remains constant regardless of how fast the object moves.

Friction coefficients and calculations

For every surface, we can determine constant factors called coefficients of friction:

• $\mu_s$ is the coefficient of static friction
• $\mu_k$ is the coefficient of kinetic friction

These coefficients allow us to calculate frictional forces when we know the normal force.

The maximum static friction is given by: $f_{s,max} = \mu_s N$

The kinetic friction is given by: $f_k = \mu_k N$

Practical applications of friction

Friction is very useful in everyday life. Without friction:

• You couldn't walk (your feet would slip)
• Cars couldn't brake effectively
• Rock climbers couldn't grip cliff faces
• Ladders would slide down walls

Early humans discovered they could create fire using friction. When you rub two pieces of wood together rapidly with sufficient pressure, the friction generates enough heat to ignite dry tinder.

Worked examples

Tension

Tension is the magnitude of the force that exists in objects like ropes, chains, and struts that provide support. For example, tension forces exist in the ropes supporting a child's swing hanging from a tree.

Remember!