Dynamics II

1. Dynamics II Contact Forces and Acceleration

2. Chapter Objectives After studying the material of this chapter, the student should be able to: • State Newton’s the three laws of motion and give examples that illustrate each law. • Explain what is meant by the term “net force”. • Use the methods of vector arithmetic to determine the net force acting on an object. • Define each of the following terms: mass, inertia, and weight. Distinguish between mass and weight. • Identify the SI units for force, mass, and acceleration. • Draw an accurate free body diagram locating each of the forces acting on an object or a system of objects. • Use free body and force diagrams help you to solve word problems. Mr. Gary

3. Key Terms and Phrases dynamicsis the study of the causes of motion force is any kind of push or pull on an object net force refers to the vector sum of all the forces acting on an object. Newton’s first law of motion is also known as Galileo’s law of inertia, where inertia refers to the tendency of an object to resist any change in its state of motion. Inertia is measured by measuring an object’s mass. Newton’s second law of motion refers to an object’s motion when a net force does not equal zero. The net force ( ) will cause an object to accelerate or decelerate. The rate of accelerationis directly proportional to the magnitude of the net force and inversely proportional to the object’s mass (m), (i.e., ). Newton’s third law of motion states that whenever one object exerts a force on a second object, the second object exerts an equal but opposite force on the first. Weight is a measure of the force of gravity on an object. Mr. Gary

4. Summary: Newton’s Laws of Motion Newton’s first law of motion is also known as Galileo’s law of inertial, where inertia refers to the tendency of an object to resist any change in its state of motion. Inertia is measured by measuring an object’s mass. Newton’s second law of motion refers to an object’s motion when a net force does not equal zero. The net force will cause an object to accelerate or decelerate. The rate of acceleration is directly proportional to the magnitude of the net force and inversely proportional to the object’s mass, m: The SI unit of force is the newton, N, and for mass it is the kilogram, kg. A net force of 1 Newton will cause a 1 kg object to accelerate at 1 m/s2, thus 1 N = 1 kg m/s2. Newton’s third law is the “action-reaction” law with which most students are familiar. However, it is necessary to be very careful in interpreting the meaning of the this law. The action force and the reaction force are equal and opposite but do not act on the same object. A good way to remember the law is the statement given in your textbook: “Whenever one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.” Mr. Gary

5. Vector A……………. Pulling Force....... Vector B……………. Normal Force; Contact Force………. Vector C………………. Force of Gravity; Weight………. Mr. Gary

6. Friction Box

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9. There’s More!! Coefficient of Friction

10. Coefficient of Friction • The coefficient of friction: • the coefficient of friction is a scalar quantity. • it is an electrostatic bond between and object and surface over which the object • passes. • this electrostatic bond is the result of the attraction of the valence electrons • that interact between a surface and a body. • mathematically it is the ratio of the normal force and friction force. • is dependent on only the “normal force.” • the answer provides us with information as to how stickiness of a surface. • the general mathematical expression is written as: • μs represents the coefficient static friction. • μk represents the coefficient of kinetic friction. Mr. Gary

11. General Mathematical Expression Mr. Gary

12. Key Terms and Phrases friction is a contact force that opposes the relative motion of two surfaces as they slide past each other. The frictional force depends on the coefficient of friction, μ, and normal force . coefficient of friction, μ,is a pure number without physical units and varies with the types of surfaces that are in contact. When the object is at rest, the coefficient of static friction μsis used to determine the magnitude of the frictional force just before the object starts to move. When the object is moving, the coefficient of kinetic friction μkis used. Mr. Gary

13. Gary’s Check List for Solving Problems • Read each problem carefully and READ IT AGAIN. • Write down conceptual meanings from the wording of each problem. (i.e., constant velocity, constant acceleration, …, etc.). Oftentimes, certain word combinations may look the same but have radically different meanings. • Record all numerical data from the problem and any data inferred (i.e., gravity, • Use both the reading and any diagram to size up a problem. • Sketch a problem from the reading and provide a free body diagram to show all forces acting on a body. • spring constant, centripetal acceleration,…, etc.). • In what units should your supporting and final answers be written. • Determine which numbers, if any, need to be converted and perform that/those operations first. Mr. Gary

14. A force of 200 N is pulled by a rope which is at an angle of 30° to the horizontal. The box has a mass of 150 kg. (a) Make a sketch of the problem. (b) Construct a free body diagram from the sketch. (c) Calculate the x, y – components for all the forces on the block. (d) Calculate the coefficient kinetic friction between the block and surface. Mr. Gary

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