Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd 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. Examples: Without a seatbelt, a man continues moving forward even though the car stops. Without being properly secured, a ladder continues moving forward even though the truck stops.

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Newton’s First Law is often called the law of Inertia. The law of inertia was first proposed by Galileo. Galileo showed that a force is not required to keep on object in motion.

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion The greater the mass, the greater the inertia. A more massive object has a greater tendency to resist changes in its state of motion. Many massive books are placed upon the head of a student. Another student drives a nail into a block of wood that sits on top of the books. The student below the books is unharmed. Why? What’s wrong on this page???

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Check Your Understanding Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose that you visit that place (just suppose) and throw a rock. The rock will gradually stop. continue in motion in the same direction at constant speed.

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Check Your Understanding Mac and Tosh are arguing in the cafeteria. Mac says that if he flings the Jell-O with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with? Explain why.

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Check Your Understanding The group of physics teachers are taking some time off for a little putt-putt golf. The 15th hole at the Hole-In-One Putt-Putt Golf Course has a large metal rim which putters must use to guide their ball towards the hole. Ms. D guides a golf ball around the metal rim When the ball leaves the rim, which path (1, 2, or 3) will the golf ball follow?

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Check Your Understanding A 4.0-kg book is moving across a friction-free surface with a constant velocity of 2 m/s. Which one of the following horizontal forces is necessary to maintain this state of motion? 0 N b. 0.5 N c. 2.0 N d. 8.0 N e. depends on the speed

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion The Meaning of Force? A force is a push or pull upon an object resulting from the object's interaction with another object. For simplicity sake, all forces (interactions) between objects can be placed into two broad categories: contact forces, and forces resulting from action-at-a-distance Contact Forces Action-at-a-Distance Forces Frictional Force Gravitational Force Tension Force Electrical Force Normal Force Magnetic Force Air Resistance Force Applied Force Spring Force

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion The Unit for Force: Force is a quantity which is measured using the standard metric unit known as the Newton. A Newton is abbreviated by a "N." To say "10.0 N" means 10.0 Newtons of force. One Newton is the amount of force required to give a 1-kg mass an acceleration of 1 m/s/s. Thus, the following unit equivalency can be stated

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Weight vs. Mass: Weight is the force due to gravity on an object. Mass describes the amount of matter or inertia that an object has. Weight (Fg) = mg = (mass x 9.8m/s2) The weight of an object changes due to the amount of mass the object has but also due to the location of the object as well. The mass of an object changes only when the amount of matter in the object changes. Mass does not change if the object changes location.

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Balanced vs. Unbalanced Forces Balanced Forces = Equilibrium Static Equilibrium or Dynamic Equilibrium (At Rest) (Constant speed) Unbalanced Forces = non-equilibrium Net force exists Acceleration exists

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Free Body Diagrams: Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation. Example: A book is at rest on a table top. Diagram the forces acting on the book.

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Free Body Diagram Practice: Example: A girl is suspended motionless from the ceiling by two ropes. Example: An egg is free-falling from a nest in a tree. Neglect air resistance.

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Free Body Diagram Practice: Example: A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. Example: A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance.

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Free Body Diagram Practice: Example: A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. Example: A college student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder.

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Free Body Diagram Practice: Example: A football is moving upwards towards its peak after having been booted by the punter. Example: A car is coasting to the right and slowing down.

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Net Force: The net force is the vector sum of all the forces which act upon an object.

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Check Your Understanding Free-body diagrams for four situations are shown below. For each situation, determine the net force acting upon the object. Situation A:_____ Situation B:_____ Situation C:_____ Situation D:_____ 0 N 5 N 0 N 15 N

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Check Your Understanding Free-body diagrams for four situations are shown below. The net force is known for each situation. However, the magnitudes of a few of the individual forces are not known. Analyze each situation individually and determine the magnitude of the unknown forces. 50 N A:_____ B:_____ C:_____ D:_____ E:_____ F:_____ G:_____ H:_____ 200 N 1100 N 20 N 300 N 300 N 50 N 300 N

a = Fnet / m Fnet = m * a Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Newton’s Second Law Newton's second law of motion pertains to the behavior of objects for which all existing forces are not balanced. The second law states that the acceleration of an object is dependent upon two variables - the net force acting upon the object and the mass of the object. a = Fnet / m Or Fnet = m * a

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Check Your Understanding An applied force of 50 N is used to accelerate an object to the right across a frictional surface. The object encounters 10 N of friction. Use the diagram to determine the normal force, the net force, the mass, and the acceleration of the object. (Neglect air resistance.) 80 N 8 kg 5 m/s2, right 40 N, right

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Check Your Understanding A rightward force is applied to a 6-kg object to move it across a rough surface at constant velocity. The object encounters 15 N of frictional force. Use the diagram to determine the gravitational force, normal force, net force, and applied force. (Neglect air resistance.) 60 N 15 N 60 N 0 N

Unit #3: Newton's Laws Lesson 1: Newton’s 1st and 2nd Law of Motion Check Your Understanding A rightward force is applied to a 6-kg object to move it across a rough surface at constant velocity. The object encounters 15 N of frictional force. Use the diagram to determine the gravitational force, normal force, net force, and applied force. (Neglect air resistance.) 60 N 15 N 60 N 0 N