Force A push or pull Have a specific size Have a specific direction Can change an object’s acceleration (speed or direction, or both) Anytime you see a change in an object’s motion, that was caused by a force

Disclaimer! Just because a force is acting on an object does NOT mean the object is moving. What is an example?

Net Force Usually multiple forces are acting on an object at once. Net force- the combination of all forces acting on an object

Your Turn- Forces in Different Directions What is the net force in this picture? 2 N to the right (always have to have a number AND direction!!!)

Forces in the Same Direction…. ….add together

Balanced Forces Meaning there is no CHANGE in the object’s motion. The object could be in motion but not changing speed or direction. Or the object could be at rest and not moving.

Unbalanced Forces… …change and object’s motion (speed or direction) …are necessary to start moving an nonmoving object. ….an object can continue to move even if one of the unbalanced forces is removed (i.e. you kick a soccer ball—it keeps moving even though your foot is no longer in contact with it)

Speed, Velocity, and Acceleration How do you know when something has moved? An object changes position over time in relation to a reference point

Speed Speed = distance traveled by an object over a certain amount of time Units: meters/sec or kilometers/hour or miles/hour Determining Average Speed: most of the time, objects don’t travel at a constant speed Average speed= total distance total time

Speed Practice Problems What is your average speed if you take 0.5 h to job 4,000 m? If the average speed of a car is 110 km/h, how long will it take the car to travel 715 km?

Velocity Velocity- speed in a certain direction Speed ≠ Velocity Velocity MUST include a reference direction You can think of velocity as the rate of change in an object’s position

Acceleration Acceleration- the rate at which velocity changes It doesn’t necessarily mean “to speed up” It means the object’s motion: Speeds up (positive acceleration) OR Slows down (negative acceleration) OR The direction of the object changes OR A combination of these occurs

Acceleration Practice Problems Average acceleration= final velocity- stating velocity Time it takes to change velocity Units= m/s/s or m/s 2 A plane passes over Point A at a velocity of 240 m/s north. Forth seconds later, it passes over point B at a velocity of 260 m/s north. What is the plane’s acceleration?

Circular Motion: Continuous Acceleration When you spin in a circle (i.e. sitting on the Earth you are always spinning in a circle), you are always changing direction, therefore you are always accelerating.

The Force of Gravity Gravity-the force of attraction between objects due to their masses. Because gravity is a force it can change the motion of an object by changing its speed, direction, or both.

Why don’t we notice the own gravity we exert? Every object has mass and therefore has the force of gravity. Why can’t you attract a pencil lying on the desk into your hand, Harry Potter style? The Earth is so massive (has so much mass than you) that you can’t overcome Earth’s gravity.

Newton’s Law of Universal Gravitation Describes the relationship between gravity, mass, and distance Part 1: Gravitational Force Increases as Mass Increases Larger mass items exert more gravity Part 2: Gravitational Force Decreases as Distance Increases Farther objects exert less gravity than closer objects

Mass v. Weight Weight- how much gravity is acting on/pulling on something It’s unit is N (newtons) (technically not pounds like your bathroom scale reports) Measured using a spring scale On earth, 1 N = approximately 100 g Mass- how much matter something has It’s unit is gram (or kg, or mg) Measured using a balance

Friction Friction- a force that opposes motion between two surfaces that are in contact. This force causes objects to slow down and eventually stop.

What affects friction 1. the amount of force pushing the surfaces together (usually caused by the masses of the objects) 2. the type of surface (rough versus smooth)

Types of Friction Static Friction- this occurs when you put an applied force on an object but it’s not enough to make it move because static friction is opposing it in equal amount. If you want to move the object, you must apply a larger applied force in order to overcome the static friction force. EX: in the Phet Simulation, when Joe pushed the file cabinet and it didn’t move even though he was pushing on it, that’s static friction working.

Types of Friction Kinetic Friction Rolling Friction: A type of moving (or kinetic) friction where 1 surface is rolling past another (example: most transportation…anything with wheels…bicycles, in-line skates, cars, trains, etc) Sliding Friction: A type of moving (or kinetic) friction where 2 surfaces are sliding past each other. Example: moving a heavy piece of furniture…applying breaks on a bike, write with a pencil, scratching,

Types of Friction Fluid Friction: occurs between layers within a fluid that are moving relative to each other. Ex: Squeaky door…with oil on hinges (sliding and rolling friction) Introduce oil, surfaces run past each other and help. OR Ice skating: the blade on your skate melts a thin layer of water and that’s what your blade slides on (this works for sledding as well!)

Ways to Reduce Friction Sometimes we want less friction i.e. the squeaky door example. You add a lubricant that reduces friction You can also make the surface smoother (have you ever gone down a slide riding on wax paper? Try it out!) You can change from sliding kinetic friction to rolling kinetic friction (you should have already noticed this difference in your lab)

Ways to Increase Friction Sometimes we want more friction Make surfaces rougher (when roads freeze we put down salt or sand to make the surface more rough and increase friction on the ice) Increase the force pushing the surfaces together (apply more mass)