Dynamics - Net Force & Q19
Dynamics Presentation - https://njctl.org/courses/science/algebra-based-physics/dynamics/dynamics-presentation-4/ Slide text: Return to Table of Contents Net Force Σ F The greek letter sigma - Σ- means -the sum of-. Sometimes Σ F is written as F net , they mean the same thing. It means you have to add up all the forces acting on an object. Net Force Let's look at the left side of this equation first. Net Force The arrow above -F- reminds you that force is a vector. We won't always write the arrow but remember it's there. It means that when you add forces, you have to add them like vectors: forces have direction , and they can cancel out. Example: A 5.0 kg object is being acted on by a 20N force to the right (F 1 ), and a 30N force, also to the right (F 2 ). What is the net force on the object? Net Force First we'll draw a free body diagram. We will discuss these in more detail later on but for now, follow these simple directions. FBDs consists of a dot, representing the object, and arrows representing the forces. The direction of the arrows represents the direction of the forces...their length is roughly proportional to their size. Newton's Second Law of Motion The first force (F 1 ) acts to the right with a magnitude of 20 N F 1 Example: A 5.0 kg object is being acted on by a 20N force to the right (F 1 ), and a 30N force, also to the right (F 2 ). What is the net force on the object? Newton's Second Law of Motion The second force, F 2 , acts to the right also, with a greater magnitude of 30N. This is drawn slightly larger than F 1 . F 1 F 2 Example: A 5.0 kg object is being acted on by a 20N force to the right (F 1 ), and a 30N force, also to the right (F 2 ). What is the net force on the object? Newton's Second Law of Motion F 1 F 2 To add vectors, move the second vector so it starts where the first one ends. The sum is a vector which starts where the first vector started, and ends where the last one ends. Example: A 5.0 kg object is being acted on by a 20N force to the right (F 1 ), and a 30N force, also to the right (F 2 ). What is the net force on the object? Newton's Second Law of Motion F 1 F 2 Σ F These free body diagrams are critically important to our work. Once done, the problem can be translated into an algebra problem. Example: A 5.0 kg object is being acted on by a 20N force to the right (F 1 ), and a 30N force, also to the right (F 2 ). What is the net force on the object? For example: A 5.0 kg object is being acted on by a 20N force to the right (F 1 ), and a 30N force, also to the right (F 2 ). What is the net force on the object? Newton's Second Law of Motion First we will define -to the right- as positive. Then we can interpret our diagram to read: Σ F = F 1 + F 2 Σ F = 20 N + 30 N Σ F = 50N to the right (we get the direction from our diagram and from our positive answer, which we defined as meaning -to the right-) F 1 F 2 Σ F 19 Two forces act on an object. One force is 40N to the west and the other force is 40N to the east. What is the net force acting on the object? Students type their answers here [This object is a pull tab] Answer Σ F = F 1 + F 2 Σ F = 40 N + (- 40 N) Σ F = 0 N Given: F 1 =40 N (east) F 2 =-40 N (west) Σ F = ?
Dynamics Presentation - https://njctl.org/courses/science/algebra-based-physics/dynamics/dynamics-presentation-4/ Slide text: Return to Table of Contents Net Force Σ F The greek letter sigma - Σ- means -the sum of-. Sometimes Σ F is written as F net , they mean the same thing. It means you have to add up all the forces acting on an object. Net Force Let's look at the left side of this equation first. Net Force The arrow above -F- reminds you that force is a vector. We won't always write the arrow but remember it's there. It means that when you add forces, you have to add them like vectors: forces have direction , and they can cancel out. Example: A 5.0 kg object is being acted on by a 20N force to the right (F 1 ), and a 30N force, also to the right (F 2 ). What is the net force on the object? Net Force First we'll draw a free body diagram. We will discuss these in more detail later on but for now, follow these simple directions. FBDs consists of a dot, representing the object, and arrows representing the forces. The direction of the arrows represents the direction of the forces...their length is roughly proportional to their size. Newton's Second Law of Motion The first force (F 1 ) acts to the right with a magnitude of 20 N F 1 Example: A 5.0 kg object is being acted on by a 20N force to the right (F 1 ), and a 30N force, also to the right (F 2 ). What is the net force on the object? Newton's Second Law of Motion The second force, F 2 , acts to the right also, with a greater magnitude of 30N. This is drawn slightly larger than F 1 . F 1 F 2 Example: A 5.0 kg object is being acted on by a 20N force to the right (F 1 ), and a 30N force, also to the right (F 2 ). What is the net force on the object? Newton's Second Law of Motion F 1 F 2 To add vectors, move the second vector so it starts where the first one ends. The sum is a vector which starts where the first vector started, and ends where the last one ends. Example: A 5.0 kg object is being acted on by a 20N force to the right (F 1 ), and a 30N force, also to the right (F 2 ). What is the net force on the object? Newton's Second Law of Motion F 1 F 2 Σ F These free body diagrams are critically important to our work. Once done, the problem can be translated into an algebra problem. Example: A 5.0 kg object is being acted on by a 20N force to the right (F 1 ), and a 30N force, also to the right (F 2 ). What is the net force on the object? For example: A 5.0 kg object is being acted on by a 20N force to the right (F 1 ), and a 30N force, also to the right (F 2 ). What is the net force on the object? Newton's Second Law of Motion First we will define -to the right- as positive. Then we can interpret our diagram to read: Σ F = F 1 + F 2 Σ F = 20 N + 30 N Σ F = 50N to the right (we get the direction from our diagram and from our positive answer, which we defined as meaning -to the right-) F 1 F 2 Σ F 19 Two forces act on an object. One force is 40N to the west and the other force is 40N to the east. What is the net force acting on the object? Students type their answers here [This object is a pull tab] Answer Σ F = F 1 + F 2 Σ F = 40 N + (- 40 N) Σ F = 0 N Given: F 1 =40 N (east) F 2 =-40 N (west) Σ F = ?