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# Force & Newton's First Law

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• electromagnetic, gravitational &amp; atomic forces
• Lack of motion or lack of CHANGE in motion does not mean forces are not being applied.
• Acceleration = change in speed (faster or slower) or direction.
• drop a paper flat vs. vertical: less air molecule resistance when vertical, so it falls faster (less friction) Is it easier to slide across a hard, polished floor or carpet?
• drop a paper flat vs. vertical: less air molecule resistance when vertical, so it falls faster (less friction) Is it easier to slide across a hard, polished floor or carpet? * Adhesion can sometimes increase friction, esp. in fluids
• ### Transcript

• 1. Forces & Motion Momentum
• 2. Newton's First Law
• Newton's First Law : The velocity of an object will not change unless the object is acted on by an outside force .
• An object is at rest will stay at rest until a force causes it to move.
• An object in motion will continue in at its current velocity until a force changes its speed or direction.
• In other words, objects resist changes in their motion (they tend to keep doing what they're doing). They have INERTIA .
• 3. Inertia
• Inertia : the resistance to changes in movement.
• Mass is a measure of inertia.
• The more mass an object has, the more resistant to changes in movement (inertia) it will have.
• Momentum is similar, but it takes mass AND velocity into account when considering how resistant to change in motion an object is.
• 4. Inertia
• 5. What is a Force?
• Force : a push or pull. Measured in Newtons.
• Contact Forces
• a force is applied as two objects touch
• child throws snowball
• horse pulls a wagon
• hammer hits a nail
• Action-at-a-Distance Forces
• a force is applied even though objects don't touch
• magnet picks up paper clip
• Moon pulls Earth's oceans
• electrons stay in orbit around an atom's nucleus
• 6. Combining Forces
• There are usually several different forces acting on an object all at once.
• The combination of all these forces is called the net force .
• 7. Balanced Forces
• Two or more forces are balanced if they cancel each other out when combined.
• If net forces are balanced, then no change in motion occurs.
• 8. Unbalanced Forces
• Multiple forces that do not cancel out are unbalanced and cause acceleration .
• 1 N (1 Newton) is the amount of force needed to give a 1-kg object the acceleration of 1 m/s 2 .
• 9.
• 10.
• If Newton's First Law is true, then why does a ball eventually stop moving after you kick it?
• ...because there are other forces acting on the object that make it slow down and stop.
• Friction
• 11. Friction
• Friction is a force resulting from the contact of two surfaces that opposes motion.
• Caused by surface interactions
• Smooth surface = less friction
• shiny/polished materials
• Rough surface = more friction
• fabric/carpet, rubber tires, sand paper
• 12.
• Fluids usually reduce friction
• water, oil, air
• atoms of fluids can slide past each other
• the &quot;thinner&quot; the fluid, the less friction there is
• 13. Fixed vs. Moving Friction
• Ever notice that it takes more effort to GET something moving than to keep it moving?
• Static Friction acts on unmoving objects.
• Due to surface roughness and adhesion (weak attraction between molecules of the surface of each object)
• Kinetic Friction acts on moving objects.
• Usually weaker than static friction
Friction ALWAYS acts in the opposite direction of motion.
• 14. Rolling Friction
• Wheels reduce the surface area that touches between the two objects.
• Instead of surfaces sliding, wheels roll, allowing a continuously new area to bear the object's weight.
• Much weaker than static or kinetic friction.
Rolling Resistance is the frictional force between a rolling object and the surface it's moving over.
• 15. Fluid Friction
• Fluid Friction is a force between a solid object and a fluid (liquid or gas).
• Examples:
• rocket flying through the air (air resistance)
• air molecules collide with the rocket, slowing it down
• swimmer crossing an Olympic pool (drag)
• water molecules slide past swimmer's skin
• water in a garden hose (viscosity)
• water molecules slide past each other and slide along the inside surface of the hose
• viscosity is a measure of how &quot;thick&quot; or resistant to flowing movement the liquid is
• 16. Measuring Friction
• Coefficient of Friction
• Tells how much frictional force must be overcome to result in motion (or how much resistance there is to movement).