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Introduction to Forces

Introduction to Forces. SHS Physics Department. What is Force?. Force (F) is any influence that COULD cause an object to accelerate. Just because an object doesn’t accelerate does NOT mean that there are no forces acting on it….

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Introduction to Forces

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  1. Introduction to Forces SHS Physics Department

  2. What is Force? • Force (F) is any influence that COULD cause an object to accelerate. Just because an object doesn’t accelerate does NOT mean that there are no forces acting on it…. ….it just means that the forces all balance out (they cancel each other out). Units We will use the SI unit for force = NEWTON (N) 1N = 1kgm/s2 (and is a very small amount of force) Another common unit (English system) is the pound (Lb) For example: F1 = 10 N right F2 = 10 N left Although there are 2 forces pushing on this box, the box won’t accelerate (because the forces cancel each other out).

  3. Types of Forces • All forces are either (1) contact forces or (2) field forces. • Contact Forces  forces caused by direct physical contact. • These are things like a push, a pull, a kick, a punch, friction, air resistance…anything where two (or more) objects are in contact. (2) Field Forces  forces that act without direct physical contact. These are often called “at a distance forces” because they can act at a distance from another object. These are things like gravity*, magnetism** and the electrostatic force. * you know that you don’t have to be in contact with the Earth to experience it’s gravitational force.** you know that two magnets can interact with each other without actually touching each other.

  4. Net Force Net Force (Fnet) the VECTOR sum of all of the forces (and or components of forces) acting in a given direction. • This is a big deal! • This is a big deal! • HEY….this is a big deal! Yes….Force IS A VECTOR! That means that all of the vector rules from chapter 3 apply (vector addition, vector resolution and the component method for vector addition). Example: A 20-N force is applied to a box to the east. A 30-N force is simultaneously applied to the box to the west. What is the net force? F1 = 20N F2 = 30N

  5. Net Force Net Force (Fnet) the VECTOR sum of all of the forces (and or components of forces) acting in a given direction. Example: A 20-N force is applied to a box to the east. A 30-N force is simultaneously applied to the box to the west. What is the net force? Fnet = F1 + F2 careful though…you aren’t just adding the numbers… you are adding the vectors. That means you have to pay attention to the directions as well. Fnet = F1 + F2 Fnet = (+20) + (-30)Fnet = -10 N This negative indicates “west” or the negative x-direction. F1 = 20N F2 = 30N In this case the forcesdo NOT cancel out. They are NOT balanced. Fnet is non-zero. This WOULD cause the object to accelerate (to the west).

  6. Net Force Net Force (Fnet) the VECTOR sum of all of the forces (and or components of forces) acting in a given direction. - Unbalanced Net Force (when Fnet≠ 0) WILL result in an acceleration. This is because the forces are NOT balanced (they do not cancel out). -Balanced Net Force (when Fnet = 0) WILL NOT result in an acceleration. When this occurs the object is said to be in equilibrium. Equilibrium is any condition in which the net force is zero. Equilibrium is any condition in which there is zero acceleration. Fnet = 0a = o Static Equilibrium is a special condition of equilibrium in which there is no acceleration (“equilibrium”) AND the object is at rest (“static”). Fnet = 0a = o v = 0

  7. Net Force Net Force (Fnet) the VECTOR sum of all of the forces (and or components of forces) acting in a given direction. One last thing…. Fnet can be called a few different things and Fnet can be written a couple of different ways…. • The Net Force is also called the resultant force (as in the “result”). • The Net Force is also called the vector sum. • Fnet can be written as: Fnet = ∑ F This is “sigma” it (mathematically) indicates to find the VECTOR sum.

  8. These definition are CRUCIAL to your application of everything dealing with forces. These notes are here for you to revisit…study these daily until you have truly internalized this information. If you are unsure of something presented in this PowerPoint PLEASE ask your teacher about it in class.We will discuss and build upon this information in class.

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