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# Force & Newton's First Law

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### Force & Newton's First Law

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