Understanding Kinematics: Key Concepts and Forces
What does kinematics study? The movement of bodies.
Concepts Needed to Study the Movement of a Body
- Reference Frame: An oriented straight line in which motion occurs. It presents an origin and an axis.
- Position: The distance between the object and the origin of the reference frame.
- Displacement: The difference between any two positions.
When does a body move? When it varies its position with respect to the origin of the reference frame.
What is velocity? The measurement of the rate at which bodies move.
What is acceleration? The measurement of the rate at which the velocity of a body varies.
What does Hooke’s Law explain? The force applied to an elastic body is directly proportional to the deformation caused. The ratio between the force applied and the elongation is constant and is called the spring constant (k).
What is friction? A force that always opposes motion.
- Fluid Friction: It occurs when a body moves in a fluid like water or air. It is due to the opposition of the fluid particles to separating. Reduce it: Changing the shape of the body.
- Solid Friction: It occurs when two solid bodies come into contact. It is due to the irregularities of the bodies that come into contact. Reduce it: Adding a liquid between the two bodies.
Characteristics of Gravitational Force:
- There is no need for contact between the bodies.
- It increases with the mass of the bodies.
- It decreases with distance.
- It always goes towards the center of the bodies.
What is electric force? The force that appears between two bodies when they are charged.
Characteristics of Electric Force:
- It does not require contact between the bodies.
- It is repulsive between charges of the same sign and attractive between charges of different signs.
- It increases with the value of the charges.
- It decreases with distance.
What is static electricity? An imbalance of electric charges within or on the surface of a material.
Similarities and Differences Between Gravity and Electric Force:
- Similarities: They act at a distance; their value decreases when the bodies are separated.
- Differences: Gravitational force is always attractive, but electric force can be attractive or repulsive. The masses are always positive, but electric charges can be positive or negative.
How can electric charge be acquired?
- By Rubbing: When two bodies in contact slide together, the charges of one go to the other.
- By Induction: When a charged body comes near a neutral body, it induces the separation of charges.
- By Contact: When a charged body touches a neutral body, it gives the neutral body part of its charge.
Characteristics of Magnetic Force:
- It acts at a distance and decreases when the distance increases.
- It can be attractive or repulsive.
- The magnet has two differentiated parts: North poles and South poles.
- It only affects certain metals like iron.
Similarities and Differences with Electric Force:
- Similarities: They act at a distance; there are two types of charges (+, -) or two types of poles (N, S).
- Differences: Electric force appears in bodies with only one charge; we cannot separate the poles; they always appear in pairs. The charges can move through matter, while the poles cannot move through matter.