Newton's Laws


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A summary of Newton's 1st, and 2nd laws.

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Newton's Laws

  1. 1. Chapter 4 section 1 The first law of motion <ul><li>If the net force acting on an object is zero, the object remains at rest </li></ul><ul><li>Or, if the object is moving, it continues in a straight line with constant speed </li></ul><ul><li>What’s another way to say “net force equals zero”? </li></ul>
  2. 2. Unbalanced Forces Change Velocity <ul><li>Skateboard at rest stays at rest until you push it. </li></ul><ul><li>The velocity increases while you are pushing it-unbalanced forces </li></ul><ul><li>Why does it slow down and stop? </li></ul><ul><li>The unbalanced frictional force acts to slow it down- change the velocity </li></ul>
  3. 3. Inertia and Mass <ul><li>All objects have a property called inertia </li></ul><ul><li>It is the tendency of an object to resist an change in its motion </li></ul><ul><li>A dirt bike rider trying to take a corner as fast as he can </li></ul><ul><li>The inertia of an object depends on the mass </li></ul><ul><li>The greater the mass the greater the inertia </li></ul>
  4. 4. Inertia and the First Law of Motion <ul><li>The first law of motion is sometimes called the law of inertia </li></ul><ul><li>What happens in a head on car crash? </li></ul><ul><li>The car stops, but the passenger not wearing a seat belt continues to move forward at the same speed the car was going. In about 0.02 s the person hits the dashboard </li></ul>
  5. 5. <ul><li>Force and acceleration are related </li></ul><ul><li>Throwing a ball with greater force has greater acceleration than a gently thrown ball </li></ul><ul><li>And </li></ul><ul><li>Mass and acceleration are related </li></ul><ul><ul><li>The greater the mass the smaller the acceleration </li></ul></ul><ul><ul><li>Softball vs baseball </li></ul></ul><ul><ul><ul><li>Assume you throw with same force, the softball has less velocity and acceleration </li></ul></ul></ul><ul><ul><li>Acceleration depends on mass as well as net force </li></ul></ul>
  6. 6. The Second Law Motion <ul><li>States that acceleration of an object is in the same direction as the net force on the object </li></ul><ul><li>Calculated a = f /m </li></ul><ul><li>Change this equation to calculate force: </li></ul><ul><li>f= m/a </li></ul>
  7. 7. Quiz <ul><li>Gravity depends on the ___________ of the objects and the _____________ between them. </li></ul><ul><li>Newton’s first law states that in order for an object’s velocity to change, forces must be _____________. </li></ul><ul><li>What is inertia? </li></ul><ul><li>What is inertia directly related to? </li></ul><ul><li>2 nd law: Acceleration depends on _________ and _____________. </li></ul><ul><li>Acceleration is in the same____________ of the net force. </li></ul><ul><li>What is centripetal force? </li></ul>
  8. 8. Projectile Motion <ul><li>Projectile is something that is shot through the air, ie a softball </li></ul><ul><li>Forward force gives it horizontal movement </li></ul><ul><ul><li>After you let go, the velocity is constant </li></ul></ul><ul><ul><li>But there is also vertical motion caused by gravity </li></ul></ul><ul><li>Important point: horizontal and vertical motion are independent, but together make the movement look curved </li></ul>
  9. 9. What if you… <ul><li>Pitched a baseball forward and at shoulder height and dropped one straight to the ground from the same height? Which one would get to the ground first? </li></ul><ul><li>They would get there at the same time- horizontal and vertical motion are independent of each other </li></ul>
  10. 10. Centripetal Force <ul><li>The net force toward the center of a curved path </li></ul><ul><li>Any force which keeps an object moving in the curved path instead of shooting off straight. </li></ul><ul><li>Examples? </li></ul><ul><li>The force of gravity </li></ul><ul><li>Friction </li></ul>
  11. 11. This week <ul><li>Thursday: review for test </li></ul><ul><li>Friday: test </li></ul><ul><li>If you miss the test, the make-up will most definitely be more DIFFICULT </li></ul><ul><li>If you plan on being gone tomorrow, get the review from me today b/c you will still have to take the test </li></ul>
  12. 12. Review <ul><li>A projectile is what? </li></ul><ul><li>Vertical motion and horizontal motion of a projectile are ____________ of each other </li></ul><ul><li>What is the net force toward the center of a curved path? </li></ul><ul><li>What is another way to say that? </li></ul><ul><li>Example? </li></ul>
  13. 13. 4.3 Newton’s 3 rd Law <ul><li>When one object asserts a force on another, the second object exerts a force that is equal in strength and opposite in direction </li></ul><ul><li>What is another way to say that? </li></ul><ul><li>For every action there is an equal and opposite reaction </li></ul>
  14. 14. Action and Reaction Forces do not cancel <ul><li>Just b/c an action produces an equal and opposite reaction, that doesn’t mean the forces balance </li></ul><ul><li>The forces are acting on different objects </li></ul>
  15. 15. Momentum <ul><li>Momentum is related to how much force is needed to change an objects motion </li></ul><ul><li>Momentum= mass multiplied by velocity </li></ul><ul><li>M = mv </li></ul><ul><li>Another way to say this would be? </li></ul><ul><li>The greater the mass and/or velocity, the greater the momentum </li></ul>
  16. 16. Law of Conservation of Momentum <ul><li>The momentum of an object doesn’t change unless its mass and/or velocity change </li></ul><ul><li>Momentum can be transferred from one object to another </li></ul><ul><li>Pool ball example </li></ul>