1 Newton’s Laws of Motion Chapter 6

2 Aristotle ( BC) believed that all object had a “natural place” and that the tendency of an object was to reside in its “natural place.” All objects were classified into categories of earth, water, air, or fire. “Natural motion” occurred when an object sought to return to its “natural place” after being moved from it by some type of “violent motion.” To keep an object moving would require a force. Causes of Motion

3 These views remained widely supported until the 1500s when Galileo Galilei ( )Galileo Galilei popularized experimentation. Isaac NewtonIsaac Newton (1642–1727) proposed that the tendency of an object was to maintain its current state of motion.

4 Sir Isaac Newton (1642 – 1727) Built on Galileo, Kepler, and others Worked out the “three laws of motion” governing the movement of all objects at all times an in all circumstances. He published them in his book Philosophiae Naturalis Principia Mathematica (mathematic principles of natural philosophy) in 1687.

5 Sir Isaac Newton –1 st Law – an object at rest or in motion stays at rest or in motion unless acted upon by an outside force * Known as the “Principle of Inertia” –2 nd Law – describes how an object accelerates or changes direction when a force is applied to it * F = ma –3 rd Law – for action there is an equal and opposite reaction

6 What is a Force? Force can be defined as a push or a pull. –(Technically, force is something that can accelerate objects.) Force is measured by N (Newton). A force that causes an object with a mass of 1 kg to accelerate at 1 m/s is equivalent to 1 Newton. 1 Newton = kg * m/s 2

7 What are the types of Forces? Type of ForceForceSymbol ContactFrictionF fric or F f ContactNormalFNFN ContactAppliedF Appl ContactSpringFSFS ContactTensionFTFT ContactThrustF thrust Long-RangeWeightF grav

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9 Contact Forces vs. Long-Range Forces –Contact Force = acts on an object only by touching it. (ie. Books resting on a desk) –Long-Range Force = forces that are exerted without contact or forces resulting from action- at-a-distance, (ie. force of gravity). –Types of Forces = see Table 6-2 in your books!

10 Forces have Agents –Each force has a specific, identifiable, immediate cause called the agent. –How to solve for agents: –1. Create a pictorial model of situation. –2. Circle the system and identify every place where the system touches the environment. –3. It is at these places that contact forces are exerted. –4. Then identify any long-range forces on the system, ie. Force of gravity (F grav ).

11 Example of Forces having Agents  A physics book resting on a desk.

12 Newton’s 1 st Law of Motion An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. (outside force)unless acted upon by an unbalanced force The velocity of an object remains constant unless acted on by an unbalanced force. “Law of Inertia”

13 Newton’s 1 st Law of Motion

14 Newton’s 1 st Law of Motion Inertia - the resistance an object has to a change in its state of motion. [Newton’s 1st Law] Equilibrium - if the net force on an object is zero, then it is said to be in equilibrium. –An object is in equilibrium when: at rest or moving at constant velocity.

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

18 Net Force After you have added and subtracted all the forces you are left with the net force acting on the object. There are several common forces acting on objects that you need to memorize: ForceSymbol FrictionF fric or F f NormalFNFN AppliedF Appl or F a NetF Net WeightF grav or F g

19 Free Body Diagrams To keep track of how all these forces are affecting a single object, it is a good idea to draw a free body diagram. A free body diagram is just a simple sketch of the object showing all the forces that are acting on it. –Draw a quick sketch of the object. –Draw an arrow showing every force acting on the object. –To calculate the net force, add any vectors acting on the same axis (x and y), making sure to pay attention to the directions.

20 Examples of Free Body Diagrams

21 Examples of F Net

22 Examples of F Net