Newton’s Laws of Motion

1. Newton’s Laws of Motion There was this fellow in England named Sir Isaac Newton. A little bit stuffy, bad hair, but quite an intelligent guy. He worked on developing calculus and physics at the same time. During his work, he came up with the three basic ideas that are applied to the physics of most motion. The ideas have been tested and verified so many times over the years, that scientists now call them Newton's Three Laws of Motion.

2. Newton’s 1st Law of Motion 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. Ex: when you ride in a car and the car comes to a stop, you body continues to move forward. Your seat belt and the friction between you and the seat stop your forward motion. This provides the unbalanced rearward force needed to bring you to a stop as the car stops.

3. 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.

4. 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.

5. Examples A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its 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.

6. Newton’s First Law is also called Law of Inertia An object's resistance to change position

7. Inertia The tendency of an object at rest to remain at rest, or, if moving, to continue moving with a constant velocity. 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).

8. Newton’s 1st 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.

9. A powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them. On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you don’t even feel it).

10. 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.

11. 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.

12. According to Newton’s 2nd Law Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).

13. What does this mean? Everyone unconsciously knows the Second Law. Everyone knows that heavier objects require more force to move the same distance as lighter objects.

14. The Second Law gives us an exact relationship between force, mass, and acceleration. It can be expressed as a mathematical equation: or FORCE = MASS times ACCELERATION F = ma

15. F = ma 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.

16. 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.

17. This is an example of how Newton's 2nd Law works: Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station,& he makes the car go 0.05 m/s/s. Using Newton's 2nd Law, you can compute how much force Mike is applying to the car. F = ma Answer = 50 newtons

18. 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?

19. 2nd 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 • F = m x a • Fill in given numbers and units • F = 1400 kg x 2 meters per second/second • Solve for the unknown • 2800 kg-meters/second/second or2800 N

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

21. Newton’s 2nd Lawproves 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 2nd 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

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25. Newton’s 3rd Law For every action there is an equal and opposite reaction.

26. 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.

27. 3rd 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.

28. Newton’s 3rd Law in Nature • Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. • The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

29. 3rd 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.

30. Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). Action-reaction force pairs make it possible for birds to fly.

31. 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).

32. Summary • An object at rest remains at rest and an object in motion maintains a constant velocity unless it experiences an unbalance force • The unbalances force acting on an object equals the object’s mass times its acceleration, or F= ma • The unit for force is the newton (n). • For every action force there is an equal and opposite reaction force.

33. 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.

34. http://science.discovery.com/interactives/literacy/newton/newton.htmlhttp://science.discovery.com/interactives/literacy/newton/newton.html • http://www.youtube.com/watch?feature=player_detailpage&v=tjIud8zYRbI