Chapter 3: Forces Types of Forces

Review  What is a force?  A force is an interaction between two objects. All forces come in pairs.  All forces are measured in units of Newtons (N)  How do forces between 2 objects interact?  According to Newton’s 3 rd Law, every action force has an equal in magnitude and opposite in direction reaction force.  Newton's Third Law Song Newton's Third Law Song

Force Vectors  Forces are represented by arrows called vectors which indicate both magnitude and direction.  The length of the arrow indicates the magnitude of the force  The direction the arrow is pointing indicates the direction of the force

Balanced & Unbalanced Forces  Objects on Earth usually experience more than one force at a given time  The combination of all the forces acting on an object is called the net force  When the net force on an object equals 0, we say the forces are balanced  Example: Because forces have direction, a 5N force upward plus a 5 N force downward, both exerted on an object, would add up to 0  Therefore the net force on an object would be 0 and the forces would be balanced  When the net force on an object does not equal 0, we say the forces are unbalanced

Bowling Ball Physics  What do I need to do to make a bowling ball start moving?  Stop moving?  Continue moving at a constant speed?

Forces and Motion  Motion does not require a cause (force)  Motion can be separated into two categories: 1. Constant velocity (no acceleration) 2. Changing velocity (acceleration)  When an object has a constant velocity, its forces are balanced  When an object has a changing velocity (speed up, slow down, change direction) it’s forces are unbalanced

Newton’s First Law  Every object moves at a constant speed in a straight line, unless an unbalanced force is applied to it  This is also referred to as inertia Tablecloth Trick

Types of Forces  There are two main types of forces:  1. Long Range Forces  The 2 objects do not need to be in contact with each other for the force to be applied  Gravitational, magnetic, electrostatic  2. Contact Forces  The 2 objects must be in contact with each other in order for the force to be applied  Normal, tension, friction, applied, buoyancy, lift

Long Range Forces: Gravitational Force  Gravitational Force (F g )  The attractive force applied by the Earth on an object  Objects are “pulled” towards the center of the Earth  Depends on the objects’ masses and the distance from the center of the Earth  The more mass and the closer the objects are, the stronger the gravitational force  Any object with mass has a gravitational force! Most objects we encounter do not have enough mass for us to notice  Family Guy Clip Family Guy Clip

Gravitational Force Examples Other examples of gravitational force:  The sun’s F g keeps all of the planets in the solar system in orbit  The planets do not crash into the sun, because they are travelling so fast  Tides on Earth are caused by the F g of the sun and the moon on Earth’s water

 Normal Force (F N )  A force applied by a surface on an object  The force is always applied perpendicular (90˚ angle) to the surface  Tension Force (F T )  A force applied by a rope, string, cable, etc. on an object  The force is always applied parallel to the rope Contact Forces

Contact Forces: Friction Force  Friction Force (F f )  A force applied by a surface with texture on an object  The force is always applied parallel to the surface in a way that resists slipping.  This can slow an object down or speed it up  i.e. sliding into home plate vs. wearing cleats to run the bases

Contact Forces: Applied Forces  Applied force (F app )  Any other type of force applied directly by you or another person on an object

Force Diagrams  Used to illustrate the forces on an object Step 1: Draw a dot. This dot represents your entire object. Step 2: Draw a vector to represent the force of gravity. Label it F g. Step 3: If the object is on a surface, draw a vector to represent the normal force. Label it F N. Step 4: If the object is being pulled/suspended by a rope, string, etc., draw a vector to represent the tension force. Label it F T. Step 5: If the object is being pushed or pulled directly by a person, draw a vector to represent the applied force. Label it F app. Step 6: If a parallel force is being applied to the object while on a surface with a texture, draw a vector to represent the friction force. Label it F f. Assume all moving objects have a friction force unless stated otherwise.