Forces Vocabulary Force Newton (unit) Contact Force Field Force Free Body Diagram (FBD) Weight Normal Force Tension Net Force Balanced Forces Unbalanced Forces “Boating on the Seine” (1879) Pierre-Auguste Renoir This topic can be found in your textbook on pp. 120-124.

A force is a push or a pull. What is a force? A force is a push or a pull.

The SI unit for force is a Newton (N). What is a force? The SI unit for force is a Newton (N).

Types of forces Contact Forces Force due to physical contact between objects.

Types of forces Field Forces Forces that act at a distance with no contact (e.g., gravity and electromagnetism.)

Forces are Vectors Forces have both magnitude and direction.

Forces are Vectors Pushing on the left…

Forces are Vectors has a different effect than pushing on the right…

Forces are Vectors which has a different effect than pulling it upward…

Forces are Vectors or pushing it downward.

Forces are Vectors Arrows are used to indicate the direction of forces. The length of the arrow indicates the relative magnitude of the force. small force to the left large force to the right

Free Body Diagrams We use what are called free body diagrams (FBD) to show all of the forces that are acting on an object. Object are represented as rectangles and each force acting on the object is represented by an arrow

Free Body Diagrams Example 1: Draw a FBD for a moving car. What forces need to be included? force of engine force of air resistance force of friction force of gravity (called “weight”) force of the ground (this is the “normal force”) Fn Fa Fe Ff Fg

Free Body Diagrams Example 2: Draw a FBD for Batman hanging on the end of a rope. What forces need to be included? Force of gravity (weight) Force of rope (called “tension”) FT Fg

Free Body Diagrams Example 2 (continued): If there are forces acting on Batman, why isn’t he moving? The forces are balanced! FT Fg

Balanced vs. Unbalanced Forces Balanced forces are forces acting on an object that are equal and opposite. This means that the net force (the forces added together) is zero. Balanced forces always cause an object to keep doing what it’s doing (stay at rest or continue moving at constant velocity, but more about this later.)

Balanced Forces Fnet = ΣF = Fwings + Fgravity = 0 N Example: Hovering bird Fwings Wings push the bird up with the exact same force that gravity pulls the bird down. Fnet = 0 N Fgravity Fnet = ΣF = Fwings + Fgravity = 0 N

Balanced vs. Unbalanced Forces Unbalanced forces occur when the net force on an object is not zero. Unbalanced forces always cause an object to accelerate (but more about this later.)

Unbalanced Forces Fnet = ΣF = Fair res + Fgravity < 0 N Example: Bungee jumper just after jumping Fair resistance Fnet = ΣF = Fair res + Fgravity < 0 N Fgravity

Finding Net Forces To find net forces, the forces need to be broken into x-direction forces and y-direction forces. Then the net x-direction force and the net y-direction force can be found separately.

Finding Net Forces Example: Draw a free body diagram of Joy Rider’s 1200 N sled being pulled on level ground with a force of 240 N and a friction force of 15 N. 1200 N 15 N 240 N 1200 N

Finding Net Forces Fnet,x= ΣF = Fpull + Ffric = 240N + (-15N) = 225 N Example: What are Fnet,x and Fnet,y for Joy’s sled? Fnet,x= ΣF = Fpull + Ffric = 240N + (-15N) = 225 N 1200 N 15 N 240 N 1200 N

Finding Net Forces Fnet,y= ΣF = Fgrav + Fnormal = 1200N + (-1200N) Example: What are Fnet,x and Fnet,y for Joy’s sled? Fnet,y= ΣF = Fgrav + Fnormal = 1200N + (-1200N) = 0 N Which way will the sled accelerate? 1200 N 15 N 240 N 1200 N

Force Summary Equation Summary Fnet = ∑F Fnet,x = ∑Fx Fnet,y = ∑Fy Measurement Symbol SI Unit force F N Equation Summary Fbuoyant Fnet = ∑F Fnet,x = ∑Fx Fnet,y = ∑Fy Fpaddle Fdrag Fgravity