2. Newton’s Laws of Motion I. Law of Inertia II. F=ma III. Action-Reaction

3. Background Sir Isaac Newton (1643-1727) an English scientist and mathematician famous for his discovery of the law of gravity also discovered the three laws of motion. He published them in his book Philosophiae Naturalis Principia Mathematica (mathematic principles of natural philosophy) in 1687. He published them in his book Philosophiae Naturalis Principia Mathematica (mathematic principles of natural philosophy) in 1687.

4. Newton’s Laws of Motion 1 st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. 1 st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. 2 nd Law – Force equals mass times acceleration. 2 nd Law – Force equals mass times acceleration. 3 rd Law – For every action there is an equal and opposite reaction. 3 rd Law – For every action there is an equal and opposite reaction.

5. What is a force? An interaction between TWO objects. For example, pushes and pulls are forces, friction, tension, gravity, and touches. We must be careful to think about a force as acting on one object from (or due to ) another object.

6. Adding Forces Forces are vectors (They have both magnitude and direction) and so add as follows: Forces are vectors (They have both magnitude and direction) and so add as follows: Examples: + = Examples: + = + = + = +3 N +6 N 3 N 0 N

7. A net force is the sum of all forces acting on an object. For instance, if the wheels of a car push it forward with 5 Newtons and drag is 3 Newtons, the net force is 2 Newtons, forward. A net force is the sum of all forces acting on an object. For instance, if the wheels of a car push it forward with 5 Newtons and drag is 3 Newtons, the net force is 2 Newtons, forward. Balance forces are two forces acting in opposite directions on an object, and equal in size. When two forces acting on an object are not equal in size, we say that they are unbalanced forces

8. 1 st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

9. Newton’s First Law is also called the Law of Inertia Inertia: the tendency of an object to resist changes in its state of motion The First Law states that all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).

10. What does this mean? Basically, an object will “keep doing what it was doing” unless acted on by an unbalanced force. If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving. It takes force to change the motion of an object.

11. Some Examples from Real Life Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion. Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion. A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion.

12. 1 st Law Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop! Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

13. 1 st Law Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever. Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

14. What is meant by unbalanced force? If the forces on an object are equal and opposite, they are said to be balanced, and the object experiences no change in motion. If they are not equal and opposite, then the forces are unbalanced and the motion of the object changes.

15. If objects in motion tend to stay in motion, why don’t moving objects keep moving forever? Things don’t keep moving forever because there’s almost always an unbalanced force acting upon it. A book sliding across a table slows down and stops because of the force of friction. If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

16. In outer space, away from gravity and any sources of friction, a rocket ship launched with a certain speed and direction would keep going in that same direction and at that same speed forever.

17. There are four main types of friction: There are four main types of friction: Sliding friction: Sliding friction: ice skating Rolling friction: Rolling friction: bowling Fluid friction (air or liquid): Fluid friction (air or liquid): air or water resistance Static friction: Static friction: initial friction when moving an object Friction is the resistance to motion of one object moving relative to another.

18. Newtons’s 1 st Law and You Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

19. Newton’s Second Law Force equals mass times acceleration. F = ma Acceleration: a measurement of how quickly an object is changing speed.

20. 2 nd Law The net force of an object is equal to the product of its mass and acceleration, or F=ma.

21. 2 nd Law When mass is in kilograms and acceleration is in m/s/s, the unit of force is in newtons (N). When mass is in kilograms and acceleration is in m/s/s, the unit of force is in newtons (N). One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second. One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.

22. 5.2 Converting newtons and pounds A force of one pound is equal to about 4.448 newtons. A force of one pound is equal to about 4.448 newtons.

23. What does F = ma say? F = ma basically means that the force of an object comes from its mass and its acceleration. Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force. Something very small changing speed very slowly will have a very weak force. Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force.

24. What does F = ma mean? Force is directly proportional to mass and acceleration. Imagine a ball of a certain mass moving at a certain acceleration. This ball has a certain force. Now imagine we make the ball twice as big (double the mass) but keep the acceleration constant. F = ma says that this new ball has twice the force of the old ball. Now imagine the original ball moving at twice the original acceleration. F = ma says that the ball will again have twice the force of the ball at the original acceleration.

25. More about F = ma If you double the mass, you double the force. If you double the acceleration, you double the force. What if you double the mass and the acceleration? (2m)(2a) = 4F Doubling the mass and the acceleration quadruples the force. So... what if you decrease the mass by half ? How much force would the object have now?

26. 2 nd Law (F = m x a) How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second? Write the formula Write the formula F = m x a Fill in given numbers and units Fill in given numbers and units F = 1400 kg x 2 meters per second/second Solve for the unknown Solve for the unknown 2800 kg-meters/second/second or 2800 N

27. Newton’s 2 nd Law proves that different masses accelerate to the earth at the same rate, but with different forces. We know that objects with different masses accelerate to the ground at the same rate. However, because of the 2 nd Law we know that they don’t hit the ground with the same force. F = ma 98 N = 10 kg x 9.8 m/s/s F = ma 9.8 N = 1 kg x 9.8 m/s/s

28. Newton’s Third Law For every action there is an equal and opposite reaction.

29. What does this mean? For every force acting on an object, there is an equal force acting in the opposite direction. Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving. There is a balanced force acting on you– gravity pulling down, your seat pushing up.

30. 3 rd Law There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.

31. 3 rd Law Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.

32. Other examples of Newton’s Third Law The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction). The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

33. 3 rd Law The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.

34. Review Newton’s First Law: Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force. Newton’s Second Law: Force equals mass times acceleration (F = ma). Newton’s Third Law: For every action there is an equal and opposite reaction.

35. Vocabulary Inertia: the tendency of an object to resist changes in its state of motion Acceleration: a change in velocity a measurement of how quickly an object is changing speed, direction or both Velocity: The rate of change of a position along a straight line with respect to time Force: strength or energy