5.1 Forces
3 min read•june 24, 2024
Forces are the invisible hands that shape our world. They're behind every push, pull, and interaction we experience. In , we study how these forces affect motion, going beyond just describing movement to understanding its causes.
Free-body diagrams are our secret weapon for visualizing forces. By drawing all forces acting on an object as arrows, we can easily see their combined effect. This simple tool helps us solve complex problems and predict how objects will behave under different forces.
Forces and Dynamics
Concept of force in dynamics
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Normal, Tension, and Other Examples of Forces | Physics View original
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- is a physical quantity that causes an object to change its motion (accelerate, decelerate, or change direction) or shape (deform or change shape)
- studies forces and their effects on motion, while studies motion without considering the forces causing it
- Forces are fundamental to understanding the behavior of objects in the physical world
- causes objects to fall towards the Earth
- opposes the motion of objects sliding against each other
- acts along strings, ropes, or cables pulling on objects
- acts perpendicular to surfaces, preventing objects from sinking into them
Construction of free-body diagrams
- A is a graphical representation of all the forces acting on an object, represented as a single point or simplified shape
- Forces are represented as vectors, with the tail at the object and the arrow pointing in the direction of the
- To construct a :
- Identify all the forces acting on the object
- Draw the object as a point or simplified shape
- Draw the force vectors, with the tail at the object and the arrow pointing in the direction of the force
- Label each force with its type (gravity, friction, tension)
- Interpreting a free-body diagram: the length of each force vector represents the magnitude of the force, the direction represents the direction in which the force acts, and the is the vector sum of all the individual forces
Newton as force unit
- The is the of force, defined as the force required to accelerate a 1 kg at 1 m/s²
- To calculate force using newtons, use the equation [F = ma](https://www.fiveableKeyTerm:F_=_ma), where is force in newtons, is mass in kilograms, and is in m/s²
- A 10 kg object experiencing an acceleration of 3 m/s² has a force of acting on it
Forces as vectors
- Forces are vector quantities with both magnitude (strength) and direction (orientation in space)
- Force vectors can be added or subtracted using vector addition rules
- To add force vectors, place them head-to-tail and draw the resultant vector from the tail of the first to the head of the last
- To subtract force vectors, add the negative of the vector being subtracted
- The net force on an object is the vector sum of all the individual forces acting on it
- If the net force is zero, the object is in and will not accelerate
- If the net force is non-zero, the object will accelerate in the direction of the net force
Kinematics vs dynamics
- studies motion (position, velocity, acceleration) without considering the forces that cause it, using kinematic equations to describe relationships between these quantities
- Dynamics studies forces and their effects on motion, building upon kinematics by introducing the concept of force and using dynamic equations like to relate forces to the motion they cause
- In summary, kinematics describes motion, while dynamics explains the causes of motion through forces
Force-related concepts
- : The resistance of an object to changes in its motion, directly related to its mass
- : The product of an object's mass and velocity, representing its motion quantity
- : The transfer of when a force acts on an object and causes displacement
- Energy: The capacity to do work, which can be stored in various forms (e.g., kinetic, potential)
- : The product of force and the time interval over which it acts, causing a change in momentum
Key Terms to Review (35)
Acceleration: Acceleration is the rate of change of velocity with respect to time. It represents the change in an object's speed or direction over a given time interval, and is a vector quantity that has both magnitude and direction.
Acceleration vector: An acceleration vector is a vector quantity that represents the rate of change of velocity of an object. It indicates both the magnitude and direction of this change.
Action-at-a-distance force: An action-at-a-distance force is a force exerted by an object on another object that is not in physical contact with it, acting over a distance through space. Examples include gravitational, electromagnetic, and nuclear forces.
Dynamics: Dynamics is the branch of mechanics that deals with the motion of objects and the forces acting on them. It focuses on how forces affect the movement of objects.
Dynamics: Dynamics is the branch of physics that deals with the study of forces and their effects on motion. It examines how objects move under the influence of various forces, taking into account factors like mass, acceleration, and friction. Understanding dynamics is crucial for analyzing the behavior of moving objects and predicting their future positions based on the forces acting on them.
Energy: Energy is the fundamental quantity that describes the ability to do work or cause change. It is the driving force behind all physical and chemical processes in the universe, from the smallest subatomic interactions to the largest-scale cosmic events. Energy can take many forms, such as kinetic, potential, thermal, electrical, and more, and it is conserved in the sense that it cannot be created or destroyed, only transformed from one type to another.
Equilibrium: Equilibrium occurs when all forces acting on an object are balanced, resulting in no net force and no acceleration. In static equilibrium, the object is at rest, and in dynamic equilibrium, it moves with constant velocity.
Equilibrium: Equilibrium is a state of balance or stability, where the forces acting on a system are in balance, and the system is at rest or in a state of constant motion. This concept is fundamental in understanding various physical phenomena, including the behavior of objects, the distribution of forces, and the stability of systems.
External force: An external force is any force that originates from outside a system and acts on the objects within that system. It can change the motion of the objects by altering their velocity or causing deformation.
F = ma: The equation F = ma represents Newton's second law of motion, which states that the force acting on an object is equal to the mass of that object multiplied by its acceleration. This fundamental principle connects the concepts of force, mass, and acceleration, illustrating how they interact in physical systems. Understanding this relationship is crucial for solving problems related to motion and dynamics in various contexts.
Field forces: Field forces are forces that act on objects without requiring physical contact. Examples include gravitational, electric, and magnetic forces.
Force: Force is any interaction that, when unopposed, will change the motion of an object. It can cause an object with mass to change its velocity (to accelerate).
Force: Force is a vector quantity that represents the interaction between two objects, causing a change in the motion or shape of the objects. It is the fundamental concept that underlies many of the physical principles studied in college physics, including Newton's laws of motion, work, energy, and more.
Free-body diagram: A free-body diagram is a graphical representation used to visualize the forces acting on an object. Each force is represented by an arrow pointing in the direction of the force with its length proportional to the magnitude.
Free-Body Diagram: A free-body diagram is a visual representation of an object or system that shows all the external forces acting on it. It is a fundamental tool used in physics to analyze the forces acting on an object and to solve problems involving Newton's laws of motion.
Friction: Friction is a force that opposes the relative motion between two surfaces in contact. It arises due to the microscopic irregularities on the surfaces, which create resistance to sliding or rolling. Friction is a fundamental concept in physics that plays a crucial role in various topics, including solving problems, understanding forces, and analyzing energy transformations.
Gravity: Gravity is a fundamental force of nature that attracts objects with mass towards each other. It is the force that keeps planets in orbit around the sun, causes objects to fall to the ground, and governs the motion of celestial bodies in the universe.
Impulse: Impulse is the product of the average force and the time interval over which it acts on an object. It is equal to the change in momentum of the object.
Impulse: Impulse is a quantity that describes the change in momentum of an object over a given time interval. It is the product of the net force acting on an object and the time interval during which that force is applied. Impulse is a fundamental concept in physics that connects the ideas of force, time, and momentum, and is essential for understanding topics such as solving problems in physics, forces, Newton's laws, and collisions.
Inertia: Inertia is the property of an object that resists changes to its state of motion. It depends solely on the mass of the object.
Inertia: Inertia is the property of an object that resists changes to its state of motion. It is the tendency of an object to remain at rest or in motion unless acted upon by an unbalanced force.
Kinematics: Kinematics is the branch of mechanics that describes the motion of objects without considering the causes of that motion. It involves parameters such as displacement, velocity, and acceleration.
Kinematics: Kinematics is the branch of physics that describes the motion of objects without considering the forces that cause the motion. It focuses on the geometric properties of motion, such as position, displacement, velocity, and acceleration, and how these quantities change over time.
Mass: Mass is a fundamental physical quantity that represents the amount of matter in an object. It is a measure of an object's resistance to changes in its state of motion, and it is a key concept in the study of mechanics and the behavior of objects under the influence of forces.
Momentum: Momentum is a vector quantity that describes the motion of an object. It is defined as the product of an object's mass and its velocity, and it represents the object's quantity of motion. Momentum is a fundamental concept in physics that is closely related to other important topics such as forces, energy, and collisions.
Net external force: Net external force is the vector sum of all external forces acting on an object. It determines the object's acceleration according to Newton's Second Law.
Net force: Net force is the total force acting on an object, taking into account both the magnitude and direction of all individual forces. It determines the object's acceleration according to Newton's second law of motion, which states that an object will accelerate in the direction of the net force. Understanding net force is crucial for analyzing how forces interact and influence motion, as it helps explain concepts like inertia, action-reaction pairs, and equilibrium conditions.
Net work: Net work is the total work done on an object, accounting for all forces acting on it. It determines the change in the object's kinetic energy.
Newton (N): The newton (N) is the unit of force in the International System of Units (SI). It is named after Sir Isaac Newton, the famous physicist who formulated the laws of motion. The newton is the force required to accelerate a mass of one kilogram at a rate of one meter per second squared.
Newton's Laws: Newton's laws are a set of three fundamental principles that describe the relationship between an object and the forces acting upon it, governing the motion of physical bodies. These laws form the foundation of classical mechanics and are essential for understanding and solving problems in physics.
Normal Force: Normal force is the support force exerted by a surface perpendicular to the object resting on it, preventing the object from falling through the surface. It plays a crucial role in balancing other forces acting on an object, particularly in scenarios involving gravity and acceleration.
SI unit: An SI unit is a standard measurement used to quantify physical quantities in the International System of Units (SI), which is the modern form of the metric system. These units provide a universal language for scientists and engineers, enabling clear communication and consistent measurements across various fields of study. SI units are fundamental in defining concepts like force, which is measured in newtons, helping to ensure accuracy and uniformity in scientific calculations and experiments.
Tension: Tension is a force that acts to pull or stretch an object, often along the length of a string, rope, or cable. It is a vector quantity, meaning it has both magnitude and direction, and it plays a crucial role in various physics concepts related to forces, motion, and equilibrium.
Vector: A vector is a mathematical quantity that has both magnitude (size or length) and direction. It is used to represent physical quantities in physics, such as displacement, velocity, acceleration, and force, where both the size and the direction of the quantity are important.
Work: Work is a physical quantity that describes the energy transferred by a force acting on an object as the object is displaced. It is the product of the force applied and the displacement of the object in the direction of the force.