Physics G11-Adv G11-ADV 4. Force 4.1 Types of Forces Contact forces Non-contact forces are forces that act between two objects that are physically touching each other are forces that act between two objects that are not physically touching each other Normal contact forces Magnetic force Frictional forces Electrostatic force Air resistance Gravitational force Mr. Adham Zewin 0505084733 1 Physics G11-Adv G11-ADV 4.2 Gravitational Force Vector, Weight, and Mass Mass weight Mass is a measure of the amount of matter an object is made out of. Weight is the force that acts on mass due to gravity and is therefore measured in kilograms (kg) measured in newtons (N). The same value everywhere Change value from place to another the gravitational force acting on the computer points downward. This direction is the direction of the gravitational force vector • the force vector of the gravitational force • The magnitude Fg is called the weight of the object. Fg =mg • 1 N ≡1 kg.m/s2 • The more massive objects seem to resist being put into motion more than the less massive ones do. This property of an object is referred to as its inertial mass Mr. Adham Zewin 0505084733 2 Physics G11-Adv G11-ADV Consider two steel balls. Ball 1 has a weight of 1.5 N. Ball 2 has a weight of 15 N. Both balls are released simultaneously from a height of 2.0 m. Which ball hits the ground first? a) Ball 1 hits the ground first. b) Ball 2 hits the ground first. c) Both balls hit the ground at the same time. d) It can’t be determined from the information provided. 4.3 Net Force Free-body diagrams Mr. Adham Zewin 0505084733 3 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 4 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 5 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 6 Physics G11-Adv G11-ADV 4.4 Newton’s Laws Newton’s First Law says there are two possible states. • an object with no net force on it: An object at rest is said to be in static equilibrium. • An object moving with constant velocity is said to be in dynamic equilibrium. Mr. Adham Zewin 0505084733 7 Physics G11-Adv G11-ADV 4.5 Ropes and Pulleys three ropes are tied together at one point, with a team pulling on each rope. Suppose team 1 is pulling due west with a force of 2750 N, and team 2 is pulling due north with a force of 3630 N. Can a third team pull in such a way that the three-team tug-of-war ends at a standstill, that is, no team is able to move the rope? If yes, what is the magnitude and direction of the force needed to accomplish this? Mr. Adham Zewin 0505084733 8 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 9 Physics G11-Adv G11-ADV 4.6 Applying Newton’s Laws Mr. Adham Zewin 0505084733 10 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 11 Physics G11-Adv G11-ADV Newton’s second Law An object with a mass of 22 kg has a force applied to it. The graph shows the change in the object’s velocity while the force is applied. How much force is applied to the object? Answer to the nearest newton Ans. 22N 5.31. An elevator cabin has a mass of 363.7 kg, and the combined mass of the people inside the cabin is 177.0 kg. The cabin is pulled upward by a cable, in which there is a tension force of 7638 N. What is the acceleration of the elevator? Mr. Adham Zewin 0505084733 12 Physics G11-Adv G11-ADV 5.33. The density (mass per unit volume) of ice is 917 kg/m3, and the density of seawater is 1024 kg/m3. Only 10.45% of the volume of an iceberg is above the water’s surface. If the volume of a particular iceberg that is above water is 4205.3 m3, what is the magnitude of the force that the seawater exerts on this iceberg? Mr. Adham Zewin 0505084733 13 Physics G11-Adv G11-ADV 4.7 Friction Force basic characteristics of friction 1. If an object is at rest, it takes an external force with a certain threshold magnitude and acting parallel to the contact surface between the object and the surface to overcome the friction force and make the object move. 2. The friction force that has to be overcome to make an object at rest move is larger than the friction force that has to be overcome to keep the object moving at a constant velocity. 3. The magnitude of the friction force acting on a moving object is proportional to the magnitude of the normal force. 4. The friction force is independent of the size of the contact area between object and surface. 5. The friction force depends on the roughness of the surfaces; that is, a smoother interface generally provides less friction force than a rougher one. 6. The friction force is independent of the velocity of the object. Mr. Adham Zewin 0505084733 14 Physics G11-Adv G11-ADV • Initially the static frictional force matches • the pulling force and we have no motion, fs = F. • When the pulling force exceeds the maximum possible static friction force, μsN, the frictional force drops abruptly to the dynamic value of μk N and stays at that constant value as the object accelerates. Static Friction Kinetic Friction Kinetic friction only occurs if the object is NOT sliding across the surface Kinetic friction only occurs if the object is sliding across the surface μs is called the coefficient of static friction. μk is the coefficient of kinetic friction. μk = 0 corresponds to a frictionless surface μs >μk μk is also less than 1 in almost all cases The maximum static friction force is greater than the force of kinetic friction The direction of the kinetic friction force is always opposite to the direction of motion of the object relative to the surface it moves on If the external force is equal to static friction force this resulting in a sudden acceleration of the object If the external force is equal to kinetic friction force the object will continue to slide across the surface with constant velocity. Mr. Adham Zewin 0505084733 15 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 16 Physics G11-Adv G11-ADV Questions Mr. Adham Zewin 0505084733 17 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 18 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 19 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 20 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 21 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 22 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 23 Physics G11-Adv G11-ADV Mr. Adham Zewin 0505084733 24
