Types of forces and free body diagrams | AP Physics 1 | Khan Academy

Summary

This video explains the concept of free body diagrams in physics, focusing on the forces acting on objects in various scenarios. The instructor discusses the forces of gravity, normal force, tension, and friction, using examples such as a block on a table, a hanging block, and an object being pulled across the ground. The video concludes with a more complex example involving a shelf supporting two objects, exploring the interaction of forces including the weight of the objects, normal forces, and tension from supporting wires. The examples help to visualize how forces interact and balance in different situations.

Takeaways

• 😀 Free body diagrams focus on the forces acting on a single object in isolation from the rest of the system.
• 😀 The force of gravity (weight) acts downward on any object on Earth and is a long-range force.
• 😀 Normal force is the upward force exerted by a surface that supports an object, balancing the object's weight in a stationary situation.
• 😀 Tension is the pulling force in a string or rope, such as when an object is hanging from a ceiling.
• 😀 Friction is a force that resists motion between two surfaces in contact and opposes the direction of applied force.
• 😀 When multiple forces act on an object, such as gravity, normal force, and friction, they balance out to maintain the object's stationary state.
• 😀 In a scenario with two ropes holding a block, the tensions in the ropes must balance the weight of the block and any other forces acting on it.
• 😀 Forces in a system can either be contact forces (like normal force and friction) or long-range forces (like gravity).
• 😀 Free body diagrams can be used to analyze complex systems, such as objects hanging or resting on surfaces with other forces acting on them.
• 😀 For objects resting on a shelf or surface, the combined downward forces (like the weight of the object and shelf) must be counteracted by an equal and opposite force from the supporting surface or ropes.

Q & A

What is a free body diagram?

A free body diagram (FBD) is a simplified illustration that shows all the external forces acting on a single object in a system. It helps in analyzing the forces and understanding the object’s motion or lack thereof.

What forces act on a stationary block sitting on a table?

For a stationary block on a table, two primary forces act on it: the downward gravitational force (weight) and the upward normal force from the table. These forces balance each other, keeping the block stationary.

What is the normal force and how is it related to the gravitational force?

The normal force is the perpendicular force exerted by a surface to support an object resting on it. It counteracts the object's weight, and for a stationary object on a horizontal surface, the normal force is equal in magnitude but opposite in direction to the gravitational force.

How does tension force act on an object hanging from a string?

When an object is hanging from a string, the tension force acts upward, counteracting the downward gravitational force. If the object is stationary, the tension force exactly balances the weight of the object.

What is friction and how does it affect the motion of an object?

Friction is a force that opposes the relative motion between two surfaces in contact. It acts in the opposite direction of applied forces, such as tension, and prevents or resists movement. In the case of a stationary object being pulled, friction balances the pulling force until the applied force exceeds friction.

What happens when a block is stationary but pulled with a force on a horizontal surface?

When a block is stationary and a force is applied to pull it, friction works against the applied force. If the pulling force is less than the friction, the block will remain stationary. If the pulling force exceeds friction, the block will begin to move.

In a scenario where a block is on a shelf with two wires holding it up, how do you analyze the forces?

In this scenario, the block’s weight applies a downward force on the shelf. The shelf's weight also acts downward. The two wires provide upward tension forces, which must balance both the shelf's weight and the block’s weight. The sum of the two tensions must equal the total downward force.

How is friction related to the roughness of surfaces or molecular interactions?

Friction arises from two factors: the roughness of the surfaces in contact, which makes it harder for the objects to slide past each other, and molecular interactions, where the molecules at the interface are attracted to each other, creating resistance to motion.

What role does the tension in the rope play when an object is hanging stationary from it?

The tension in the rope provides an upward force that counteracts the downward gravitational force on the hanging object. When the object is stationary, the tension force equals the gravitational force, maintaining equilibrium.

Why is it important to only focus on the forces acting on a single object in a free body diagram?

Focusing only on the forces acting on a single object in a free body diagram allows for a clear analysis of that object’s motion or lack thereof. By isolating the object, we can identify the forces in different directions and understand how they balance out to determine the object’s behavior.