Concept of a Force

What is a Force? Usually think of a force as a push or pull. A force can deform, stretch, rotate, or compress a body. It makes an object accelerate. A force is the result of an interaction between two objects – An object doesn’t possess a force – An object cannot produce a force on its own. It needs a partner Vector quantity May be contact or field force

Contact and Field forces Contact forces require contact between the 2 objects interacting together. Field forces don’t require contact. They act at a distance.

The Four Fundamental Forces There are four kinds of forces in nature. These forces are called fundamental forces: – Gravitational force: This is the force that holds us onto the Earth. The weakest force among the four fundamental forces. Infinite range – Electromagnetic force: This is the principle that keeps atoms together. This is the force which exists between all particles which have an electric charge. Many everyday experiences such as friction and air resistance are due to this force. This is also the resistant force that we feel, for example, when pressing our palm against a wall. strong force Infinite range – Weak nuclear force: responsible for the radioactive decay processes inside the atomic nuclei – Strong nuclear force: responsible for keeping quarks together inside protons and neutrons, and keeping protons and neutrons inside atomic nuclei. Very strong but short range – The gravitational and electromagnetic forces can be experienced in our daily life. They are also called the familiar forces which are the electromagnetic and gravitaional forces. Similarly, the strong force and the weak force are called the unfamiliar forces.

Common Examples of Forces Weight Tension Normal reaction forces Drag forces Upthrust forces Frictional forces

Weight The weight of an object is the result of the interaction between the object and the planet earth. So, the weight of an object depends not only on the object but also on the planet it is interacting with. – Your weight on Earth is different that your weight on the Moon or Jupiter. Symbol: w or Fg Weight is vector quantity. Its direction is always vertically downward, towards the center of earth. W= m.g – m is the mass of the object. SI Unit is Kg. – g is the gravity of the planet (also called gravitational field strength). SI unit is Nkg -1 or m.s -2 – Unit of weight is N or Kgms -2

Weight versus Mass Mass is the property of the object, Weight is not. Mass of an object is constant everywhere in the universe, weight varies according to the location of the object.

Tension Exists in ropes, strings, springs, etc. Tension force in a string is the result of electromagnetic interactions between the molecules of the material making up the string. A tension force in a string is created when two forces are applied in opposite directions at the ends of the string.

More on Tension Consider the string hanging from the ceiling from one end and attached to a mass M at the other end. At the point of support at the ceiling, the tension force pulls down on the ceiling. – at the same time, the ceiling pulls upwards on the string by the same amount of force. At the point of contact with the mass, the tension force acts upwards on the mass. – at the same time, the mass pulls down on the rope by the same amount of force.

More on Tension Most of the time, we neglect the mass of the string. – The tension is the same at all points in the string. The direction of the tension force is always along the string. Non-stretched string has zero tension force.

More on Tension

Normal Reaction Force The normal force is the support force exerted upon an object that is in contact with another stable object/support. if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. The direction of the force is always perpendicular to the surface exerting the force. The origin of the force is electromagnetic force.

Drag Forces Drag forces are forces that oppose the motion of a body through a fluid (gas or fluid). Its source is also electromagnetic force. Air resistance forces acting on a car are examples of drag forces. A marble thrown in oil or in honey experiences drag forces. Drag forces depend on: – The shape and size of the object, – velocity and inclination – Properties of the fluid: viscosity

Upthrust Any object placed in a fluid experiences an upward force called upthrsut, buoyant force, or lift. If an object floats in water, the upthrust will be equal to the weight of the object. If upthrust is less than weight, the object sinks. If upthrust is more than weight, the object rises: the example of hot air balloon or helium balloon.

Formula 1- Formula for Success: Aerodynamics

Frictional Forces Frictional forces are also electromagnetic in origin. The oppose the motion of the object. Friction arises when two surfaces move past each other. This type of friction is called kinetic friction. Friction forces can also exist when the two surfaces are not sliding across each other: static friction.

Static Friction A heavy box rests on the floor: static friction force exists between the surfaces of the floor and the box to prevent the box from being set into motion. The static friction force balances the force that you exert on the box such that the stationary box remains at rest. When exerting 5 Newton of applied force on the box, the static friction force has a magnitude of 5 Newton. Suppose that you were to push with 25 Newton of force on the large box and the box were to still remain in place. Static friction now has a magnitude of 25 Newton. Then suppose that you were to increase the force to 26 Newton and the box finally budged from its resting position and was set into motion across the floor. The box-floor surfaces were able to provide up to 25 Newton of static friction force to match your applied force. Yet the two surfaces were not able to provide 26 Newton of static friction force. The amount of static friction resulting from the adhesion of any two surfaces has an upper limit. In this case, the static friction force spans the range from 0 Newton (if there is no force upon the box) to 25 Newton (if you push on the box with 25 Newton of force). 25 N is the maximum static friction force. Once in motion, pushing the box becomes much easier. The type of friction that you need now to overcome is kinetic friction which is less than the maximum static friction.

Free-Body Diagram Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation. The term “free” is because the body is shown on its own, free of its surroundings The object is represented by a box. The forces are represented by arrows from the center of the box outward in the direction that the force is acting. The size of the arrow in a free-body diagram reflects the magnitude of the force. Each force arrow in the diagram is labeled to indicate the exact type of force.