Chapter 5 5.1 Forces

IMPORTANT DEFINITIONS Force – a push or a pull All forces have both a size (a number) and a direction

5.1 The cause of forces A force is a push or pull, or an action that has the ability to change motion. Forces can increase or decrease the speed of a moving object. Forces can also change the direction in which an object is moving.

5.1 How are forces created? Forces are created in many ways. For example, your muscles create force when you swing a baseball bat.

5.1 Unit conversions The newton (N) is a smaller unit of force than the pound (lb). If one pound of force equals 4.448 newtons, then a 100 lb person weighs 444.8 newtons.

Types of Forces Contact Forces- those types of forces that result when the two interacting objects are perceived to be physically contacting each other. Non-Contact Forces - 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.

CONTACT FORCES Applied force - An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.

CONTACT FORCES Normal Force - The normal force is the support force exerted upon an object that is in contact with another stable object. For example, 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. 

CONTACT FORCES Tension force - The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.

CONTACT FORCES Spring force - The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position.

CONTACT FORCES Frictional Force - The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. Though it is not always the case, the friction force often opposes the motion of an object.

CONTACT FORCES Air Resistance force - The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object.

NON-CONTACT FORCES Gravity - The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed "downward" towards the center of the earth.

NON-CONTACT FORCES Electrical Force -  For example, the protons in the nucleus of an atom and the electrons outside the nucleus experience an electrical pull towards each other despite their small spatial separation. 

NON-CONTACT FORCES Magnetic Forces- two magnets can exert a magnetic pull on each other even when separated by a distance of a few centimeters. 

5.1 Gravity The force of gravity on an object is called weight. At Earth’s surface, gravity exerts a force of 9.8 N on every kilogram of mass. g=9.8N/kg

5.1 Weight vs. mass Weight and mass are not the same. Mass is a fundamental property of matter measured in kilograms (kg). Weight is a force measured in newtons (N). Weight depends on mass and gravity.

5.1 Calculating weight

Weight depends on mass and gravity A 10-kilogram rock has the same mass no matter where it is in the universe. On Earth, the10 kg. rock weighs 98 N.. On the moon, the same rock only weighs 16 N.