 # Forces & motion 2012.

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Forces & motion 2012

force Define: Push or a pull
Describe: a force is described by its direction in which it acts and its strength Strength of force is measured in the Newton (N) Direction and strength can be represented by an arrow Force is in direction of the arrow If you push a door you exert a force in a different direction than if you pull the door

Combining Forces - The Nature of Force net force = The combination of all forces acting on an object If you exert a force of 120 N on a desk and your friend exerts a force of 150 N in the same direction. What is the net force on the desk?

Unbalanced Forces - The Nature of Force Unbalanced forces = result in a net force and cause motion.

Balanced Forces Balanced forces = do not cause motion. Net force = 0
- The Nature of Force Balanced forces = do not cause motion. Net force = 0 Balanced forces in opposite directions? How will that affect motion?

Questions A force is a push or pull. What is a force?
- The Nature of Force Questions Question Answer A force is a push or pull. What is a force? What happens when forces combine? Forces combine to produce a net force.

**Two main forces – friction and gravity**

Friction A force that two surfaces exert on each other when they rub together Acts in the opposite direction of the object’s motion Depends on 2 factors: 1. how hard the surfaces push together 2. Types of surfaces involved

friction Measured in Newtons
Friction will eventually cause everything to stop Rough surfaces = more friction

Important uses Walking Drive a car Slowing down – brakes Warm up your
hands Erase a mistake You could not walk without the friction between your shoes and the ground. As you try to step forward, you push your foot backward. Friction holds your shoe to the ground, allowing you to walk. Consider how difficult it is to walk on slippery ice, where there is little friction.

Problems with Friction
Make movement difficult - Go to push a box across the floor or drag something Fall off your bike and stop yourself by sliding on your knees Wastes energy: use extra fuel Creates heat (conservation of energy energy changes form – heat) Wears things out (moving parts, tires, shoes, use oil to reduce)

gravity Force that pulls object toward each other 9.8 m/s2
You are attracted to all the objects around you and all the objects around you are attracted to you.

Law of Gravitational Attraction
Any two objects are attracted to each other Two factors that affect the gravitational attraction between objects: 1. mass 2. distance

Continued We don’t notice our gravitational attraction between other objects because these forces are small compared to the Earth’s attraction.

Gravity: mass & distance

Mass Amount of matter in an object Units for mass: kilograms

weight - Friction and Gravity The force of gravity on a person or object at the surface of a planet is known as weight. I have a revolutionary weight loss plan. A trip to the moon, where you would weigh 1/6 of what you do on Earth Why wouldn’t this weight-loss plan have the effect most people want (the amount of matter or mass of a person’s body would not change)

1. What is the scientist’s mass?
2. What is exerting the greater gravitational force – Earth or the moon? 3. Which is a more accurate measurement – mass or weight? What is the scientist’s mass? What is exerting the greater gravitation force

Free fall Only force acting on an object is gravity
In free fall gravity is an unbalanced force  causes the object to accelerate

Free fall Acceleration:
change in direction All objects regardless of their mass accelerate at the same rate.

Free Falling & velocity
For every second a falling object’s velocity increases by 9.8 m/s Velocity: speed with a direction (down, SE, etc.) Start at 0 sec = 0 m/s 1 sec = 9.8 m/s 2 sec = 19.6 m/s 3 sec = ? 29.4 m/s

Air Resistance - Friction and Gravity Falling objects with a greater surface area experience more air resistance.

Terminal velocity Greatest velocity that can be attained by an object in free fall Air resistance and velocity = balanced Humans = ~ 124 miles per hour

Comparing and Contrasting
Gravity Friction Pulls objects toward one another Effect on motion Opposes motion Types of surfaces involved, how hard the surfaces push together Depends on Mass and distance Measured in Newtons Newtons