Force and Motion

ForceA 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 ForceUsually 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 DirectionsWhat 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 AccelerationHow do you know when something has

moved? An object changes position over time in

relation to a reference point

SpeedSpeed = distance traveled by an object over a

certain amount of timeUnits: meters/sec or kilometers/hour or

miles/hour

Determining Average Speed:most of the time, objects don’t travel at a

constant speedAverage speed= total distance

• total time

Speed Practice ProblemsWhat 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?

VelocityVelocity- speed in a certain directionSpeed ≠ VelocityVelocity MUST include a reference

direction

You can think of velocity as the rate of change in an object’s position

AccelerationAcceleration- the rate at which velocity

changesIt doesn’t necessarily mean “to speed up”

It means the object’s motion:Speeds up (positive acceleration) ORSlows down (negative acceleration) ORThe direction of the object changes ORA combination of these occurs

Acceleration Practice ProblemsAverage acceleration= final velocity- stating

velocity• Time it takes to change velocity

• Units= m/s/s or m/s2

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 AccelerationWhen 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 GravityGravity-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 GravitationDescribes the relationship between

gravity, mass, and distancePart 1: Gravitational Force Increases as

Mass IncreasesLarger mass items exert more gravity

Part 2: Gravitational Force Decreases as Distance IncreasesFarther objects exert less gravity than

closer objects

Mass v. WeightWeight- how much gravity is acting

on/pulling on somethingIt’s unit is N (newtons) (technically not

pounds like your bathroom scale reports)

Measured using a spring scaleOn earth, 1 N = approximately 100 g

Mass- how much matter something hasIt’s unit is gram (or kg, or mg)Measured using a balance

FrictionFriction- a force that opposes motion

between two surfaces that are in contact.This force causes objects to slow down

and eventually stop.

What affects friction1. 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 FrictionStatic 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 FrictionKinetic 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 FrictionFluid 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 FrictionSometimes 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 FrictionSometimes 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)