AP Fizziks

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). 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). Units We will use the SI unit for force = NEWTON (N) 1 N = 1 kgm/s 2 (and is a very small amount of force) Another common unit (English system) is the pound (Lb)

All forces are either (1) contact forces or (2) field forces. (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. (1)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.

This is a big deal! HEY….this is a big deal! Net Force (F net )  the VECTOR sum of all of the forces (and or components of forces) acting in a given direction. 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). F1 = 20NF2 = 30N 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?

Net Force (F net )  the VECTOR sum of all of the forces (and or components of forces) acting in a given direction. F1 = 20NF2 = 30N 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? F net = 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. F net = F1 + F2 F net = (+20) + (-30) F net = -10 N This negative indicates “west” or the negative x-direction. In this case the forces do NOT cancel out. They are NOT balanced. F net is non-zero. This WOULD cause the object to accelerate (to the west).

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

Net Force (F net )  the VECTOR sum of all of the forces (and or components of forces) acting in a given direction. One last thing…. F net can be called a few different things and F net 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. F net can be written as: F net = ∑ F This is “sigma” it (mathematically) indicates to find the VECTOR sum.