Lesson 2 Force, Its Representation, and Mass vs. Weight Newton’s Laws

What is Force? A force is a push or pull upon an object resulting from the object’s interaction with another object. Whenever there is an interaction between two objects, there is a force upon EACH object. All forces can be placed into two categories: Contact forces Forces resulting from action at a distance Image from

What are forces? Contact forces are those types of forces that result when two interacting objects are perceived to by physically contacting each other. Action at a distance forces are those types of forces that result even when the two interacting objects are NOT in physical contact with each other yet are able to exert a push or pull despite their physical separation. Image from Tutor Circle

What are forces? Contact forces include frictional force, tension force, normal force, air resistance force, applied force, and spring forces. Action at a distance forces include gravitational forces, electric forces, and magnetic forces Image from Henderson (2001)

What are forces? Force is measure using the Newton (SI unit) A Newton is abbreviated “N” A Newton is the amount of force required to give a 1 kg mass an acceleration of 1 m/s/s. Force is a vector quantity (magnitude and direction) You must describe forces with magnitude ex. 10 N and direction ex. Down, up, etc.

Free Body Diagrams A free body diagram is a vector diagram that uses arrows to represent magnitude and direction. The size of the arrow represents the magnitude and the direction of the arrow represents the direction that the force is acting. Image from Studying the Physical World

Types of Forces Applied Force – a force that is applied to an object by a person or another object. Example – a person pushing a desk/box across the floor. The person applies a force to push the desk. Gravity Force (weight) – the force with which the earth, moon, and other massively large objects attract another object towards itself. Example – Earth, moon, or other planets around the sun

Types of Forces Normal Force – the support force exerted upon on object that is in contact with another stable object. Example – a book resting upon a surface, the surface exerts a force up on the book. Friction Force – the force exerted by a surface as an object moves across it or makes an effort to move across it. Example – a book sliding across the surface of a desk. Friction results from the two surfaces being pressed togther closely causing intermolecular attractive forces between molecules of different surfaces.

Types of Forces Air Resistance Force – a special type of frictional force that acts upon objects as they travel through the air. Example - skydiver Tension Force – the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. Spring Force – the force exerted by a compressed or stretched spring upon any object that is attached to it. Example – stretched spring

Mass vs. Weight Mass is the amount of matter that is contained in an object. Weight is the force of gravity acting upon the object. Mass is related to how much stuff there is and weight is related to the pull of Earth upon that stuff. The mass of an object will be the same no matter where in the universe it is located.

Mass vs. Weight The mass of an object will be the same whether it is moving or not moving and it will be the same whether it is being pushed upon or not. Weight is measured in Newtons and will vary according to where you are in the Universe. Weight depends upon which planet is exerting the force and the distance the object is from the planet. Weight depends upon the value of g. On Earth g = 9.8 N/kg

Mass vs. Weight On the moon, g = 1.7 N/kg The value of g is inversely proportional to the distance from the center of the planet. Jupiter and Saturn are larger planets thus they have higher g values. Image from Strobel, 2001

Friction Force Two types of friction force – sliding force or static force. Sliding friction results when an object slides across a surface. Example – pushing a box across a floor. The formula for sliding friction force is Sliding force = μ sliding x F norm The symbol μ sliding represents the coefficient of sliding friction between the two surfaces. The coefficient value depends upon the nature of the surfaces that are in contact with each other.

Friction Force Static friction forces are when the surfaces of two objects are at rest relative to one another and a force exists on one of the objects to set it into motion relative to the other object. Example: Apply 5 Newtons of force to move a box. If the box does not move then the box exerted 5 Newtons of static friction force. If you then apply 25 Newtons of force and the box does not move then the box again has 25 Newtons of static friction force. F frict-static ≤ μ frict static x F norm

Works Cited Davis, D. (2002). Newton’s Laws of Motion. Retrieved from Henderson, T. (2001). Weightlessness in Orbit. Retrieved from om/Class/circles/u6l4d.html om/Class/circles/u6l4d.html Strobel, N. (2001). Mass vs. Weight. Retrieved from Studying the Physical World. (n.d.). Forces are Central to Physics Retrieved from Tutor Circle. (2010). Non Contact Force. Retrieved from exhibiting-non-contact-force.JPG exhibiting-non-contact-force.JPG