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Chapter-2 1 FORCE AND MOTION Ms. Eman Alqurashi Lecture Outlines 2 Objective 1: Explain the force and identify kind of forces. Objective 2: Analyze and calculate the net force. Objective 3: Explain the concept of Inertia. Objective 4: Differentiate between mass and weight. Objective 5: Differentiate between balanced and unbalanced forces. Ms. Eman Alqurashi 2.1 Force 3 Force is a push or pull on an object to change its position. Ms. Eman Alqurashi The force has both magnitude and direction so it is a vector quantity . Like any vector quantity, force is represented by an arrow. The length of the arrow gives the magnitude of the force and the direction of the arrow gives the direction of the force. Magnitude of the force The force is measured in Newton (N) Ms. Eman Alqurashi 4 direction of the force. Forces between objects which are not in physical contact are called non-contact forces. Example The force between earth and the moon. Ms. Eman Alqurashi 5 2.2 Some Common Forces 6 2.2.1 Earth’s Gravitational Force (weight) Is a measure of the amount of gravitational force acting on an object having mass (m). It is directed downward toward the center of earth and represents the weight of the object w . Fg = W= mg Where ( g=9.8 m/s²) Ms. Eman Alqurashi Credit: Fundamental of physics,” By Halliday, Resnick, and Walker,” 6th Ed. John Willey &Sons, 2001. 2.2.2 Tension Force Tension force is a contact force (pull) provided by a string, rope, or wire on an object. The direction of the tension T in the direction of the rope. Credit: “Physics for scientists and engineers a strategic approach,” by Randall D. Knight Pearson-Addison Wesley 2004. Ms. Eman Alqurashi 7 2.2.3 Normal Force Normal force n is force provided by a surface (table or wall, for example) against an object that is pressing on the surface. Normal force is always points vertically upward to the surface. Credit: “Physics for scientists and engineers a strategic approach,” by Randall D. Knight, Pearson- Addison Wesley 2004. Ms. Eman Alqurashi 8 2.2.4 Friction Force Friction force f is a force provided by a surface (table or wall, for example) against any attempt to move an object along the surface in contact with. Credit: “Physics for scientists and engineers a strategic approach,” by Randall D. Knight, Pearson-Addison Wesley 2004. Ms. Eman Alqurashi 9 Examples: Identify the forces acting on the object Ms. Eman Alqurashi 10 Friction a) Always acts parallel to the surface of contact and opposite to the direction of motion. b) Acts in the direction of motion. c) Is smaller when starting than moving. d) In reality, there is no friction force. Ms. Eman Alqurashi 11 When a person begin to move forward, which force allows him to accelerate a) Weight. b) Normal force. c) Air resistance. d) Friction. Ms. Eman Alqurashi 12 2.2.5 Combining Forces: The Net (resultant) Force Fnet 13 The net force F is a single force resulted from combing all the forces such that it represents their net effect. The direction of the net force is always in the direction of the bigger force. Find the net force? Ms. Eman Alqurashi Net force: Force: Is the combination of all forces acting on that object. Is the push or a pull on an object. 5N 5N 5N 5N (5 + 5 = 10 N) 5N (5 _ 5 = 0 N) 10N (10 _ 5 = 5 N) 10N 0N 5N 2.4 Mass and Inertia What is inertia? Is the tendency of an object to remain at rest if it’s already at rest , or to keep moving if it’s already moving. Or Is the property of matter that resists change in motion. An object with a large amount of inertia requires a large amount of force to start or stop. An object with a small amount of inertia requires a small amount of force to start or stop. Conclusion: Mass is a measure of inertia. Mass is measured in kilograms (kg) Note: The more mass an object has have. the more inertia it will The less mass an object has the less inertia it will have. Mass The mass of an object is the amount of matter it contains. m=F a Mass is measured in grams(g), kilograms(kg). Weight Is a measure of the amount of gravitational force acting on an object having mass (m).(vector) w = mg Where ( g=10m/s²) Weight is measured in pounds, Newton's(N). mass and weight are directly proportional to one another. Example: A cube of butter has a mass of about 0.1 kg . Find its weight? W = mg = 0.1 × 10 =1 N a g acceleration of the objet unit(m/s²). acceleration due to the gravity or gravitational acceleration unit(m/s²). F = ma Force of the object unit (N). W =mg Gravitational force or weight unit(N). Remember: Gravitational force (weight) is proportional to mass double the mass and the gravitational force will be doubled which means the ratio of weight to mass is always the same . ( w / m = mg /m = g) Is your weight in the moon the same as your weight in the earth? what about your mass? Example 2.3 How much mass is required to balance the object on the left side, which weighs 600N? Ms. Eman Alqurashi 21 Equilibrium Equilibrium for objects at rest (static equilibrium) An object is in mechanical equilibrium when the sum of all forces (the net force) acting on it is zero ∑F=0 Equilibrium for moving objects (Dynamic equilibrium) Moving objects also can be in equilibrium when the net force is zero ∑F=0 this is the case when an object moves with constant velocity Identify the kind of equilibrium? Credit: “Physics for scientists and engineers a strategic approach” by Randall D. Knight Pearson-Addison Wesley 2004 Ms. Eman Alqurashi 23 At rest or or Net force is zero or The object can be in equilibrium if it is Moving with uniform motion or Moving with constant velocity Acceleration is zero or If an object is moving with constant velocity along a straight line, then a) No forces acting on the object. b) Single constant force acting on the object in the direction of motion. c) Single force acting on the object in the opposite direction. d) A net force of zero acting on the object. Ms. Eman Alqurashi 25 If net force acting on an object is not zero, then the object will accelerate. a) True b) False a=0 is the condition for mechanical equilibrium. a) True b) False Ms. Eman Alqurashi 26 Balanced and unbalanced forces 27 balanced forces have Unbalanced forces have Fnet 0 Fnet 0 F3 F1 F2 Example: The sketch shows a painter’s staging in mechanical equilibrium . The person in the middle weights 250N, and the tension in both ropes are 200N. What is the weight of the staging? ∑F=0 ∑F = 200 + 200 – 250 – W =0 W = 200 + 200 – 250 =150 N Ms.Eman Alqurashi 28