Force A push or pull exerted on an object.

Forces… Forces are vectors Net Force Have both magnitude and directions Net Force Vector sum of all forces acting on a body When the net force on an object is zero, the object is in equilibrium In otherwords, the acceleration is zero.

Force & Motion Forces cause accelerations Change in velocity Speed and/or direction Consider the book on the table. In order for the book to move, an unbalanced force has to act on the book.

Systems When considering how a force affects motion, it is important to identify the object of interest. This object is called the system. Everything around the object that exerts forces on it is called the external world. The identifiable cause is called an agent.

Contact Forces A contact force exists when an object from the external world touches a system and thereby exerts a force on it. Tension Friction Applied Normal

Field Forces Field forces are exerted without contact. Electrical Also known as non contact forces or action at distances forces Magnetic Gravitational Electrical

Free Body Diagram Free Body Diagram: a physical model that shows all the forces acting on the system. Each force is drawn as a vector leading away from the object. *Must be drawn for every problem

FBD Example A car accelerating to the right experiencing friction. Normal Friction Applied Weight

Newton’s First Law Also known as the law of inertia!!! An object in motion, stays in motion; an object at rest stays at rest unless acted on by a net force. Also known as the law of inertia!!!

Inertia…

Inertia An object’s resistance to change its state of motion. An object’s mass is a measurement of the object’s inertia. The more mass an object has, the more resistance it will have to change its state of motion.                                                        

Mass vs. Weight Mass Weight Amount of matter present in an object Constant – same everywhere in the universe A measurement of an object’s inertia Measured using a BALANCE Weight Force due to gravity W = mg Changes as the acceleration due to gravity changes Measured using a SCALE

Newton’s 2nd Law Acceleration is directly proportional to the net force acting on the object and inversely proportional to the object’s mass. F = ma

F = ma F = force m = mass a = acceleration Measured in Newtons Measured in kilograms a = acceleration Measured in m/s2

Newton’s 3rd Law For every action force there is an equal and opposite reaction force.

Interactions All forces result from interactions between objects. Action-Reaction force pairs act on different objects. Action-Reaction forces do NOT cancel out because they act on different objects.

Action Force: earth pulls on you Reaction Force: you pull on earth Action and Reaction Forces act on Different Masses Consider you and the earth Action Force: earth pulls on you Reaction Force: you pull on earth

Action Force: tire pushes on road Reaction Force: road pushes on tire

Action Force: rocket pushes on gases Reaction Force: gases push on rocket

Consider hitting a baseball with a bat Consider hitting a baseball with a bat. If we call the force applied to the ball by the bat the action force, identify the reaction force. (a) the force applied to the bat by the hands (b) the force applied to the bat by the ball (c) the force the ball carries with it in flight (d) the centrifugal force in the swing (b) the force applied to the bat by the ball