LETS GET STARTED This lesson will cover: How to draw a Free Body Diagram The basic forces that impact motion All 3 of Newtons Laws, and how they relate mathematically AND conceptually BUT FIRST….Get your notebooks out, and any possible HW’s on projectiles.

TOPIC FOUR! Dynamic Forces Last topics: How motion occurs (accelerations, velocities, etc) This topic: What CAUSES motion? Answer: Forces. A force is a push or a pull applied by one object and experienced by another.

The Language…DON’T WRITE We have to be REALLY careful about our language with forces. It is really easy to misinterpret situations if you confuse the language/wording. Objects don’t ‘HAVE’ forces, they ‘Experience’ forces. Example: The heavier person experiences a larger force from Earth than the lighter person.

FREE BODY DIAGRAMS…DON’T WRITE When describing forces acting on an object, we always start by drawing a FREE BODY DIAGRAM that has: 1) A labeled arrow representing each force, with each arrow starting at the object and pointing in the direction the force acts. 2) A list on the side of all forces acting on the object, including the object applying the force AND the object experiencing the force. In this class, the only forces that can act without touching are…

SOME FORCES…YEAH, WRITE THIS! Weight: force of a planet on an object near that planet. Equation: Weight = Normal Force: A force of a surface on an object. Acts perpendicular to the surface.

SOME FORCES…YEAH, WRITE THIS! Force of Friction: A force of a surface on an object. Acts parallel to the surface. Kinetic friction: Friction force acting when something is moving on the surface. Acts in opposite direction of motion. Static friction: Friction force between two surfaces that are not moving relative to one another. The coefficient of friction: μ, is how rough or smooth the surface is. Surfaces have both kinetic and static coefficents. Equation for both FRICTIONS.

FREE BODY DIAGRAM EXAMPLE A car moving right on the Beltway applies the brakes and skids to a stop. Draw and label the forces (not components) that act on the car to slow it down.

This Example 1) Draw the object you are focusing on Weight: Force of Earth on the car Fn: Force of ground on car 1) Draw the object you are focusing on 2) If it is on/near Earth, the Earth pulls on the car. It is NOT gravity, the force is called ‘Weight’. 3) All other forces must be the result of contact with the car. What is the car touching? GROUND! The ground exerts a normal force perpendicular to surface. Since its sliding on ground, kinetic friction is there…in the opposite direction of motion. Fn Ff: Force of ground on car Ff Car Weight

And we are done! Wait… the car is moving right! So there must be a force to the right making it go forward! NOPE. Car is not in contact with anything pushing it forward. AND the car is going forward because of some force that happened in the past. We only care about NOW. And at that moment, it is slowing down because there is an unbalanced force in the opposite direction of motion.

YOU TRY On the homework you picked up, lets try 41

NEWTONS FIRST LAW OF MOTION “ An object in a state of constant motion (including zero velocity) will continue in that state unless acted on by an unbalanced, or net, force. That constant motion occurs when the forces are in Equilibrium. That means that the forces experienced by an object are balanced, so combined they are equal and in the opposite direction. “When in equilibrium, stuff keeps doing what its doing” -Jochmans

NEWTONS FIRST LAW OF MOTION The property of objects to: Stay at rest until it experiences an unbalance of force to make it move Keep going at a constant velocity until an unbalance of force to speed it up OR slow it down Is called ‘INERTIA’. Which is the resistance of an object to changing its state of motion. Represented by MASS, in Kg. Newtons 1st law is often referred to as the Law of Inertia. You don’t need an unbalanced force to keep an object still or moving at constant velocity, its own inertia does it.

LETS DEMO IT! 1) RING 2) SHOVE

NEWTONS SECOND LAW OF MOTION This law is all about what happens when an object is NOT in equilibrium; AKA when an unbalanced force/forces are experienced by an object. Lets call the unbalanced force/forces the net force, represented by ΣF or Fnet Treat each direction separately. So objects can have net forces in X and or Y or both. Add forces in the same direction, subtract forces that are in the opposite direction.

NEWTONS SECOND LAW OF MOTION So Net Forces, which are the amount of unbalance experienced by an object, will overcome the inertia and change the objects motion. So Fnet will overcome the mass of an object to change its motion. A net force acting on a mass causes it to accelerate in the direction of the net force. The larger the net force, the larger the acceleration. Yet the larger the mass, the larger the net force needed to make it accelerate.

NEWTONS SECOND LAW OF MOTION Fnet = ma Fnet is a vector with magnitude and direction So Fnet has units of kg*(m/s2) which we call a Newton (N) If there is no change in motion occurring in a direction, the net force for that direction is zero. That does NOT mean that there are no forces in that direction! If an object is moving along a surface, the acceleration perpendicular to the surface is zero. Therefore the net force perpendicular to the surface is zero.

Our Example Two ropes pull a 50kg car. One has a tensional force of 300N to the right. The other has a tensional force of 200 to the left. Ignoring friction, find the acceleration of the cart as it moves on a table. 1) Draw a FBD 2) Since moving on a table, it is only moving in X. So Y is in equilibrium Ignore the Y forces unless we need them to calculate something in X. (like we did with time for projectiles) Weight: Force of Earth on the car Fn: Force of ground on car Fn F2: Force of rope on car left 200N F1: Force of rope on car right 300N F2 F1 50 kg Car Weight

Our Example In X direction: Force to RIGHT is greater than Force to LEFT. So car will accelerate to the RIGHT. + and – are relative. Fnet = 300N – 200N, will = 100N to the right. Our goal is acceleration, and we know the amount of unbalance. Fnet = ma 100N = ma, mass is 50kg. Acceleration is 2m/s2 to the right.

Our Example Question1: Can you determine if the car was moving right at that time? NO, NO, NO! The cart could be moving LEFT, but this net force is slowing it down. An acceleration does not guarantee a direction of motion. Question 2: Can you determine the normal force on the car? YES! The motion in X means the Y forces are in balance. So IN THIS CASE, the normal force is equal to the Weight since they are the only 2 Y forces. Weight = 50*10, = 500N down. Fn is then 500N up.

LETS TRY A HW

NEWTONS 3rd LAW Sometimes called the Law of Action/Reaction. The force on Object A from Object B is equal in magnitude and opposite in direction from the force of Object B on Object A. Called companion forces or action-reaction forces. Easy to screw up… What is the companion force to the normal force here? Its NOT weight, it’s the force of the car acting on the ground Weight: Force of Earth on the car Fn: Force of ground on car Fn 50 kg Car Weight

WHAT DO I MEAN? 1) FLY VS TRUCK 2) HITTIN’