How to Understand Forces Using a Free-Body Diagram

A Free-Body Diagram (FBD) is a helpful tool in physics. It shows us the forces acting on an object. This makes it easier to understand how different forces interact. This is important for figuring out how things move according to Newton's laws.

Types of Forces

Forces can be split into two main groups:

1. Contact Forces: These happen when objects touch each other. Here are some common examples:

   • Frictional Force ($F_f$): This force acts along the surfaces that are touching and tries to stop movement. For example, friction between two wooden surfaces usually ranges from about 0.3 to 0.5.
   • Normal Force ($F_N$): This is the support force that pushes up against the weight of an object when it is resting on another surface.
   • Tension Force ($T$): This is the pulling force that goes through a string or rope when it is pulled at both ends.

2. Non-Contact Forces: These forces work from a distance without touching the object. Key examples include:

   • Gravitational Force ($F_g$): This is the force that pulls objects toward each other, like how the Earth pulls everything down. On Earth, the strength of this pull is about $9.81\, \text{m/s}^2$. We can figure this force out using the formula: $F_g = m \cdot g$ Here, $m$ is the weight in kilograms and $g$ is the pull of gravity.
   • Electromagnetic Force: This force includes both electric and magnetic effects that can either pull together or push away charged particles.
   • Nuclear Force: This force holds the center of atoms together and is not usually discussed in year 10 physics.

How to Create a Free-Body Diagram

1. Choose the Object: Start by focusing on the object you want to analyze.

2. Draw the Object: Use a simple shape like a box or a dot to represent the object.

3. Show the Forces:

   • Draw arrows for all the forces acting on the object. The direction of the arrow shows which way the force is pointing, and the length of the arrow shows how strong the force is.
   • Clearly label each force (for example, $F_g$, $F_N$, $F_f$, $T$).

4. Use Newton’s Second Law: After drawing the forces, apply the following equation: $F_{\text{net}} = m \cdot a$ Here, $F_{\text{net}}$ is the total force acting on the object, $m$ is the mass, and $a$ is how fast the object is speeding up or slowing down.

Example

Let’s look at a box sitting on a flat surface with these forces acting on it:

• The weight ($F_g$) pushing down,
• The normal force ($F_N$) pushing up,
• The frictional force ($F_f$) pushing sideways if we try to move the box.

Finding Force Values

If the box weighs $2\, \text{kg}$, we can find the weight: $F_g = 2 \cdot 9.81 = 19.62 \, \text{N}$ (downward)

If the normal force is balancing the weight (the box isn’t moving up or down), then $F_N = 19.62 \, \text{N}$ (upward). If the box is sitting on a surface with friction that is 0.4: $F_f = \mu \cdot F_N = 0.4 \cdot 19.62 = 7.848 \, \text{N}$ (this force resists the box moving).

By drawing out and visualizing the forces in this way, students can better understand how they work together in different situations.