Forces and Free Body Diagrams Physical Science

Types of Forces Contact Forces Friction Normal force AKA- support force Tension force Applied force

Friction Ff Force is opposite direction of the motion Type of surface and weight affect the intensity of it Always parallel to surface Air drag or Air Friction (Fair) when things fall in the air.

Normal Force/ Support Force Fn or Fs force exerted upon an object which is in contact with another stable object Perpendicular force

Tension Force Ft Force transmitted through a string, rope or wire

Applied Force Fa Force which is applied to an object by a person or another object Ex: pushing a desk

Types of Forces Fg Fg = mass x gravity Fg = mg Distance Forces Force of Gravity Fg Force at which the earth, moon or other objects attract another object towards itself How the acceleration of gravity affects the mass of an object. Fg = mass x gravity Fg = mg Other types of Distance forces magnetism electrostatic

Free Fall: the constant acceleration of an object moving only under the influence of gravity is g The acceleration caused by gravity is 9.8m/s2 (or until you get the hang of it you can use 10 m/s2) agravity= 9.8m/s2 or g= 9.8m/s2 If there was no air, all objects would fall at the same speed Doesn’t depend on mass After 1 second falling at 10 m/s After 2 seconds 20m/s http://www.youtube.com/watch?v=KD p1tiUsZw8

http://www.youtube.com/watch?v=PHxvMLoKRWg&feature=related

So, Fg and W are the same thing! All objects experience a force of gravity downward towards the center of the earth Force of gravity is AKA weight! So, Fg and W are the same thing! So what’s the difference between mass and weight?

800 N Gravity On the earth, a person’s weight is 800 N. I love physics! 800 N

Gravity On the moon, his weight is around 130 N. Why? 130 N

Mass vs Weight Mass is the amount of matter in an object. It’s measured in grams or kilograms. General rule: If you are given a force, you will divide by 9.8 Weight is the amount of acceleration of gravity acting on an object’s mass. It’s measured in Newtons. General rule: If you are given a mass, you will multiply by 9.8

Calculating weight (in Newtons not Pounds!!) To calculate the weight of an object you multiply the object’s mass by the gravitational field strength wherever you are. Weight (N) = mass (kg) x gravitational field strength (N/kg) Or W= m x g Fg = m x g Common gravity values: On Earth: g = 9.8 m/s/s or about 10 m/s/s On the moon: g = 1.67 m/s/s

Examples w = mg or Fg = mg 1. Find the force exerted by gravity on a 1kg object. 2. Find the force exerted by gravity on a 5 kg object. 3. The force of 98 N is exerted by gravity on an object. What is the mass of the object? 4. The mass of an object is 70kg. What is the force of gravity?

Free Body Diagrams (aka Force Diagrams) Show magnitude and direction of all forces acting on an object

Draw force diagrams for the following: If an object’s net force is zero, then it is either A or B. If A. The object is not moving or at rest on a surface. (So if an object is being supported on a surface the weight(Fg) and the normal force (Fn) are always the same!) If B. It is moving at constant speed. If on a surface: (So if an object is at constant speed on a surface then the force applied (Fa) and the force of friction are always the same!)

Practice Problem 1 w= mg A 55kg person is sitting on a chair. a. What is their weight (in Newtons!) b. What is the normal force (support force)? c. Draw a force diagram showing the forces. d. What is the net force on the block?

Practice Problem 2 A block with a mass of 6 kg is being dragged across a table at a constant speed. It requires 3N of force to drag it because of friction. Draw a force diagram showing the forces involved. What is the net force on the block?