The tendency of objects to resist change in their state of motion is called inertia  Inertia is measured quantitatively by the object's mass.  Objects will undergo changes in motion only in the presence of a net (unbalanced) force Chapter 4: Force and the Laws of Motion Net force,: ALL of the forces acting on one, unique object. These forces are often summarized in a freebody diagram. If the forces cancel each other, then the net force acting on the body is equal to zero. When this happens, the object is said to be in a state of equilibrium: static equilibrium occurs when the object is at rest; dynamic equilibrium occurs when the object is moving at a constant velocity or speed 1

Static or Dynamic equilibrium (Newton’s First Law) Types of Force (most common) weight normal friction tensions applied forces A man stands on a scale inside a stationary elevator N mg Reading on scale 2

N mg An object at rest must have no net force on it. If it is sitting on a table, the object exerts a downward force mg on the surface of the table. The surface of the table exerts an upward force on the block, called the normal force. It is exactly as large as needed to balance the force from the object. ** Important: The normal force is always perpendicular to the surface (90 degree) m The reason is for Newton’s 3 rd Law. 3

F1F1 F2F2 Action/Reaction Forces Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first. Newton’s Third Law F1F1 F2F2 F 1 = Box pushes on table F 2 = Table pushes on box F 1 = Box pushes on table F 2 = Table pushes on box Then is how we draw the FBD – not the action/reaction forces 4

More Free Body Diagrams – Equilibrium 1.A 20kg box at rest on a 4.5m table. 2.A 20kg box at rest on the floor 3. Draw the free body diagram for the following problems – don’t solve just the FBD. #84-85, 90, 98 5

Finding Net Force in equilibrium 1.A 400 N car is being pulled by a tow truck with 600N force (constant velocity). If the force of friction is 50N, what is the net force on the car. Draw the FBD 2.Complete #84, 87 pg Finding component of Net Force. 1.A box is being pulled with a net force of 20N at angle of 15 degrees. Find the x and y component of the net force. 2.Complete pg. 882.

1.Forces of 10.0 N north, 20.0 N east, and 15.0 N south are simultaneously applied to a 4.00-kg mass. Find the net force in x direction and y direction. Then find total net force. 2.Force of 20.0N up, 40.0N down, 30.0N left are applied to 5.00kg object. Find the net force in x direction and y direction. Then find total net force.

Objects on incline in equilibrium   N mg 8

Incline problem – equilibrium. 1.A 10kg box is sliding down incline (angle of 10 degrees) with a constant speed of 20 m/s. a.Draw the FBD. b.Find the force of friction c.Find the normal force. 2.A 50kg box is sliding down incline (angle of 25 degrees) with a constant speed of 20 m/s. a.Draw the FBD. b.Find the force of friction c.Find the normal force.

Incline problem – equilibrium Continue… 1.A 50N box is sliding down incline (angle of 15 degrees) with a constant speed of 50 m/s. a.Draw the FBD. b.Find the force of friction c.Find the normal force. 2.A 100N box is sliding down incline (angle of 50 degrees) with a constant speed of 5 m/s. a.Draw the FBD. b.Find the force of friction c.Find the normal force.

What Happens if you accelerate? * Velocity is not constant* Example: A net force 400N is pulling a 500kg truck. What is the acceleration of the truck? Example: If you kick a ball with a net force of 100N horizontal that accelerates at 2.5 m/s 2, what is the mass of the ball?

What about Friction? Static Friction: the force that resist the sliding of motion between two surfaces that are in contact and at rest. Kinetic Friction: the force that opposes the movement of two surfaces that are in contact and are sliding over each other. (Constant velocity or constant acceleration) 12