Introduction to Force and Its Effects

TL;DR

Force is a push or a pull that can change an object's motion or shape. We experience forces constantly, from walking to seeing objects fall. Understanding forces helps explain why things move or stay still in the world around us.

1. The Mental Model

Imagine you're trying to move something heavy. You either push it or pull it, right? That push or pull is a force. Forces are always at play, even when things seem still, like a book resting on a table.

2. The Core Material

A force is essentially an interaction that, when unopposed, will change the motion of an object. This change can be:

• Starting motion: Kicking a stationary soccer ball.
• Stopping motion: A catcher stopping a baseball.
• Changing speed: Pressing the accelerator to go faster, or the brake to slow down.
• Changing direction: Steering a car around a corner.
• Changing shape: Squishing a ball of play-doh.

Forces are vector quantities, meaning they have both a magnitude (how strong they are, e.g., 10 Newtons) and a direction (e.g., upwards, to the left). We measure force in Newtons (N). One Newton is roughly the force you need to lift a small apple.

Let's look at some common types of forces you'll encounter:

Gravitational Force (Weight)

This is the force of attraction between any two objects with mass. On Earth, this is what pulls everything downwards. Your weight is the gravitational force exerted on you by the Earth. It always acts downwards, towards the center of the Earth.

Normal Force

When an object rests on a surface, the surface pushes back on the object. This push is called the normal force, and it always acts perpendicular (at a 90-degree angle) to the surface. For example, a book on a table experiences an upward normal force from the table, balancing the downward gravitational force.

Frictional Force

This force opposes motion (or attempted motion) between two surfaces in contact. Friction can slow things down, like when you slide a box across the floor, or it can help things move, like giving your shoes grip when you walk. It acts parallel to the surface.

Applied Force

This is any force directly applied to an object by a person or another object, like pushing a door open or pulling a rope.

Tension Force

This force is transmitted through a string, rope, cable, or wire when it's pulled tight by forces acting from opposite ends. It always pulls along the length of the string/rope.

Here's how forces can affect an object:

graph TD
    A["Object at Rest"] --> B{Force Applied?};
    B -- Yes --> C{Direction of Force vs. Motion?};
    B -- No --> D["Remains at Rest"];
    C -- Same Direction --> E["Speeds Up"];
    C -- Opposite Direction --> F["Slows Down/Stops"];
    C -- Perpendicular --> G["Changes Direction (Turns)"];
    C -- Other Angle --> H["Changes Speed AND Direction"];
    A --> I{Force Changes Shape?};
    I -- Yes --> J["Deforms Object"];
    I -- No --> K["Shape Unchanged"];

3. Worked Example

Imagine you're pushing a shopping cart down an aisle.

1. You apply a force: You push the cart forward with, say, 50 N of force. This is your applied force.
2. Friction acts: The wheels and the floor create friction, opposing your motion. Let's say the frictional force is 20 N backward.
3. Gravity acts: The Earth pulls the cart downwards. If the cart's mass is 20 kg, its weight (gravitational force) is about 200 N downwards (we'll learn how to calculate this precisely later).
4. Normal force acts: The floor pushes upwards on the cart, balancing its weight. So, there's a normal force of 200 N upwards.

The forces changing the cart's forward motion are your 50 N applied force forward and the 20 N frictional force backward. Since your push (50 N) is greater than the friction (20 N), the cart will accelerate (speed up) in the direction you're pushing.

4. Key Takeaways

• A force is a push or a pull that can start, stop, speed up, slow down, or change the direction of an object's motion, or deform its shape.
• Forces are vector quantities, meaning they have both a magnitude (strength) and a direction.
• The standard unit of force is the Newton (N).
• Common forces include gravitational force (weight), normal force, frictional force, applied force, and tension force.
• When forces are balanced, an object's motion doesn't change; when unbalanced, its motion changes (it accelerates).

5. Now Try It

Think about an object you use every day, like a cell phone. Identify at least three different forces acting on your phone at different times or in different situations. For each force, describe its direction and what effect it has (or tries to have) on the phone. For example, what forces act on it when it's sitting on a desk, or when you pick it up, or if it slides off a tilted surface?