Types Of Forces & Free Body Diagrams: Videos & Practice Problems

Understanding forces is crucial in physics, particularly through Newton's laws. Key forces include applied force (F_a), tension (T), weight (W), normal force (N), and friction (f). Free body diagrams (FBD) visually represent these forces acting on an object, aiding problem-solving. For example, in calculating acceleration (a) using F = m a, one must consider net forces, including weight, normal, and friction forces. This foundational knowledge is essential for analyzing motion and interactions in various physical scenarios.

Types of Forces

In the study of forces, understanding the five most common types is essential for solving problems effectively. The first type is the applied force, denoted as F_a, which occurs when an object is directly pushed or pulled. For instance, when you push a box, the force is represented by an arrow pointing away from the object's center in the direction of the push.

The second type is tension, represented by the symbol T. This force arises when a rope, string, or cable is pulled. If you pull on a rope attached to a box with a force of 5 newtons, the tension in the rope also equals 5 newtons, acting in the direction of the pull, illustrated by an arrow from the object's center.

The third force is the weight force, symbolized as W, which is essentially the force of gravity acting on an object. This force always points towards the center of the Earth, typically represented by a downward arrow in problems unless specified otherwise, such as in space.

Next, we have the normal force, indicated by N. This force acts as a reaction force when two surfaces are in contact. For example, a box resting on the ground experiences a weight force acting downwards, while the ground exerts an upward normal force that is perpendicular to the surface. This normal force can also act at angles, such as when pushing a box against a wall, where it still remains perpendicular to the surface.

Lastly, the friction force, represented by f, occurs when two rough surfaces slide against each other. This force opposes the direction of motion, which is crucial in understanding why objects eventually come to a stop when sliding. For example, if a box slides across a table, the friction force acts in the opposite direction to its movement, slowing it down.

In summary, recognizing these forces—applied, tension, weight, normal, and friction—is vital for analyzing and solving problems involving motion and interactions between objects. Each force has its unique characteristics and direction, which play a significant role in the overall dynamics of the system.

A tire swing hangs from a tree branch by a rope. Identify all the forces acting on the tire.

• A. Weight and Normal Force
• B. Weight and applied force
• C. Weight and tension
• D. Weight, tension, and normal force

Answer: (C)

You push a couch across a carpeted (rough) living room floor. Identify all the forces acting on the couch.

• A. Weight, normal, friction
• B. Weight, applied, friction
• C. Weight, applied, normal, tension
• D. Weight, applied, normal, friction

Answer: (D)

Free-Body Diagrams

Understanding free body diagrams (FBDs) is crucial in physics, particularly when analyzing the forces acting on an object. A free body diagram is a simplified representation that illustrates all the forces acting on a single object, typically depicted as a dot or a box. This visual tool helps organize and clarify the forces involved in a problem, making it easier to solve complex scenarios.

To create a free body diagram, start by identifying the object of interest and representing it as a dot. Next, consider the various forces acting on the object. The first force to include is the weight force, which always acts downward due to gravity, directed towards the center of the Earth. This force can be calculated using the equation:

\( F_{weight} = m \cdot g \)

where \( m \) is the mass of the object and \( g \) is the acceleration due to gravity (approximately \( 9.81 \, \text{m/s}^2 \)).

Next, identify any applied forces or tensions. An applied force occurs when an external agent pushes or pulls the object, while tension is the force transmitted through a rope or string when it is pulled tight. It’s important to avoid double counting forces; for example, if a hand is pulling a rope, the force is transmitted as tension rather than being counted as an applied force directly on the object.

Another force to consider is the normal force, which acts perpendicular to the surface in contact with the object. This force counteracts the weight force when the object is resting on a surface. Lastly, if the object is in contact with a rough surface, frictional forces must be included. Friction opposes the direction of motion and is directed opposite to the applied force.

Once all forces are identified, they are represented as arrows emanating from the center of the object, with the length of each arrow proportional to the magnitude of the force. This clear representation allows for easier analysis of the forces involved.

To calculate the acceleration of the object, apply Newton's second law, which states:

\( F_{net} = m \cdot a \)

Here, \( F_{net} \) is the net force acting on the object, which can be determined by summing the forces in each direction. For example, if analyzing horizontal motion, the net force can be calculated by subtracting the frictional force from the applied force. In vertical motion, the net force is the difference between the upward tension and the downward weight force.

By systematically drawing free body diagrams and applying Newton's laws, one can effectively analyze and solve problems involving forces and motion, leading to a deeper understanding of the physical principles at play.

Drawing a FBD

In this scenario, we analyze a block being pushed against a vertical wall, which is rough, using a free body diagram to identify the forces acting on the block. The block is pushed at a 45-degree angle, and it is sliding upwards, indicating an upward velocity.

To construct the free body diagram, we start by representing the block as a dot or box. The first force to consider is the weight force, which always acts downward towards the center of the Earth. This force can be denoted as W and is directed vertically downwards.

Next, we identify the applied force, which is the force exerted on the block at a 45-degree angle. This force can be labeled as F and is directed at 45 degrees from the horizontal. While it is not necessary to illustrate the angle in the diagram, it is important to acknowledge its direction.

Since there are no ropes or strings involved in this situation, we do not have any tension forces acting on the block. The next force to consider is the normal force, which arises from the contact between the block and the wall. The normal force acts perpendicular to the surface of the wall, pointing outward from the wall.

Finally, we must account for friction, which occurs due to the roughness of the wall. Friction always acts in opposition to the direction of motion. Given that the block is moving upwards, the frictional force will act downwards, opposing this upward motion.

In summary, the free body diagram for the block includes the following forces: the weight force (W) acting downwards, the applied force (F) at a 45-degree angle, the normal force acting outward from the wall, and the frictional force acting downwards. Understanding these forces is crucial for analyzing the motion of the block and applying Newton's laws of motion effectively.

Here’s what students ask on this topic:

What are the different types of forces in physics?

In physics, the main types of forces include:

• Applied Force (F_a): This occurs when a person or object directly pushes or pulls another object.
• Tension (T): This force is transmitted through a string, rope, or cable when it is pulled tight by forces acting from opposite ends.
• Weight (W): This is the force of gravity acting on an object, directed towards the center of the Earth.
• Normal Force (N): This is the support force exerted upon an object in contact with another stable object, acting perpendicular to the surface.
• Friction (f): This force opposes the motion of two surfaces sliding past each other and acts parallel to the surfaces in contact.

How do you draw a free body diagram?

To draw a free body diagram (FBD), follow these steps:

1. Identify the object: Represent the object as a dot or a simple shape like a box.
2. Draw the forces: Draw arrows from the object's center to represent all forces acting on it. The length and direction of each arrow should correspond to the magnitude and direction of the force.
3. Label the forces: Clearly label each force with its type (e.g., F_a for applied force, T for tension, W for weight, N for normal force, and f for friction).
4. Check for completeness: Ensure all relevant forces are included, such as gravitational force, normal force, applied force, tension, and friction.

What is the difference between normal force and friction force?

The normal force (N) and friction force (f) are both contact forces but differ in their directions and roles:

• Normal Force (N): This force acts perpendicular to the surface of contact. It is a reaction force that prevents objects from passing through each other. For example, a book resting on a table experiences a normal force from the table supporting its weight.
• Friction Force (f): This force acts parallel to the surface of contact and opposes the relative motion between two surfaces. There are two types of friction: static friction (preventing motion) and kinetic friction (opposing ongoing motion). For example, when you slide a box across the floor, friction acts in the opposite direction of the box's movement.

How do you calculate acceleration using a free body diagram?

To calculate acceleration using a free body diagram (FBD), follow these steps:

1. Draw the FBD: Identify and draw all forces acting on the object.
2. Resolve forces: Break down forces into components along the x and y axes if necessary.
3. Sum the forces: Use Newton's second law, $F=ma$, to sum the forces in each direction. For example, in the x-direction: $\sum F=ma$.
4. Solve for acceleration: Rearrange the equation to solve for acceleration (a). For example, if the net force in the x-direction is $\sum F=12$ N and the mass (m) is 2 kg, then $a=\frac{12}{2}=6$ m/s².

What is the role of friction in motion?

Friction plays a crucial role in motion by opposing the relative movement between two surfaces in contact. There are two main types of friction:

• Static Friction: This force prevents an object from starting to move. It acts when an object is at rest and must be overcome to initiate motion.
• Kinetic Friction: This force opposes the motion of an object that is already moving. It acts in the opposite direction of the object's velocity, causing it to slow down and eventually stop.

Friction is essential for everyday activities, such as walking, driving, and holding objects, as it provides the necessary grip and resistance to control motion.