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

- The Nature of ForceCombining Forcesnet force = The combination of all forces acting on an object

- The Nature of ForceUnbalanced ForcesUnbalanced forces = result in a net force and cause motion.

- The Nature of ForceBalanced ForcesBalanced forces = do not cause motion.

Net force = 0

Questions

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

What happens when forces combine?

Forces combine to produce a net force.

Question Answer

- The Nature of 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

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

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

- Friction and Gravity

weightThe force of gravity on a person or object at the surface of a planet is known as weight.

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?

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/s2 sec = 19.6 m/s 3 sec = ?

- Friction and GravityAir ResistanceFalling 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

Friction Gravity

Comparing and Contrasting

Effect on motion Opposes motion Pulls objects toward one another

Depends onTypes of surfaces involved, how hard the surfaces push together

Mass and distance

Measured in Newtons Newtons