Forces and Free Body Diagrams Physical Science

Types of Force Contact Forces Friction Normal force/ support force Tension force Applied force

Friction-Ff Force is opposite direction of motion Surface and weight Always parallel to surface Air drag

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

Action at Distance 1. magnetism 2. electrostatic 3. Gravity Fg : force at which the earth, moon or other objects attract another object towards itself

Free Fall: the constant acceleration of an object moving only under the influence of gravity is g The acceleration caused by gravity is 10 m/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 20 http://www.youtube.co m/watch?v=KDp1tiUsZ w8

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

Force of gravity is weight All objects experience a force of gravity downward towards the center of the earth Force of gravity is weight 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

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 = m x g 1. The force exerted by gravity on 1kg= 3. The force exerted by gravity on ____kg= 98 N 4. The force exerted by gravity on 70kg= ________N 1. The force exerted by gravity on 1kg= 2. The force exerted by gravity on 5 kg = _____ N

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!) If not on a surface:

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?

Free Body Diagrams Show magnitude and direction of all forces acting on an object