Transcript Document
Chapter 11: Forces 11.1: Forces change motion 11.2: Force and mass determine acceleration 11.3 Forces act in pairs 11.4 Forces transfer momentum 11.2 Forces and mass determine acceleration Newton’s second law relates to force, mass, and acceleration Slide a milk container: use more force if full Force = mass x acceleration F = m x a F = ma To give two objects with different mass (m) the same acceleration (a), must apply a different force (F). Combining Forces In the same direction (think tailwind) In opposite directions (headwind) Newton’s Second Law The acceleration of an object increases with increased force and decreases with increased mass F = ma Also: the direction in which an object accelerates is the same as the direction of the force Calculating Acceleration If a team pulls with a combined force of 9000 N on an airplane with a mass of 30,000 kg, what is the acceleration of the airplane? What do you know? What do you want to find out? Formula? Substitute: Calculate: Check units: Mass and Acceleration If the same force acts on two objects, the object with less mass will have the greater acceleration F = ma Calculating Mass: model rocket it accelerating at 2 m/s2. Force on it is 1 N. What is the mass of the rocket? Forces can change the direction of motion Force can either speed up or slow down the motion of an object, and can also make an object change direction Recall: if an object changes direction, it is accelerating Can change the direction of an object without changin its speed Forces can change the direction of motion Centripetal Force Think spinning in a circle with a friend: arms get tired – force used to change the direction of your motion to keep in a circle Any force that keeps an object moving in a circle Points toward the center if the circle Ball on a string stays in a circle due to the force on the string changing direction, yet always along the string toward the center Centrifugal Force Points towards the outside of a circle The centrifugal force does not act on the body in motion; the only force acting on the body in motion is the centripetal force The centrifugal force acts on the source of the centripetal force to displace it radially from the center of the path Thus, in twirling a mass on a string, the centripetal force transmitted by the string pulls in on the mass to keep it in its circular path, while the centrifugal force transmitted by the string pulls outward on its point of attachment at the center of the path The centrifugal force is often mistakenly thought to cause a body to fly out of its circular path when it is released; rather, it is the removal of the centripetal force that allows the body to travel in a straight line as required by Newton's first law Circular Motion and Newton’s Second Law Skater in photo p55: must apply more force for a faster direction change If you know the size of the centripetal force acting upon the object, you can find its acceleration A greater acceleration requires a greater centripetal force A more massive object requires a greater centripetal force to have the same circular speed as the less massive object Force and acceleration are always directed towards the center of the circle Newton's second law can describe the exact effects of what happens when things like the air bag and crumple zone are put to use in an automobile. Newton's second law can be easily expressed by one equation alone. Acceleration - Force / Mass. This law governs acceleration and is simple to understand. You can lower the acceleration which is produced by your body in a collision by using safety features like crumple zones and air bags. "The acceleration a of a mass m by an unbalanced force F is directly proportional to the force and inversely proportional to the mass, or a- F / m." This is the second law of motion. "Every action has an equal and opposite re-action." This is Newton's third law and it also has plays a role in exactly how both airbags and crumple zones work. Newton's third law would probably be the most important of all of them. When you have an impact with your car, not only does your car hit another object, but the object pushes back on your car. Since the crumple zone on your car is made soft to absorb impacts, it will take the brunt of the collision so that the time it takes the car to come to a complete stop is spread out over a longer period of time. Not only may this save other people's lives it may also save money in low speed collisions.