Presentation on theme: "Concept Summary Batesville High School Physics. Newton’s Second Law If there is a net force on an object, the object accelerates. Its acceleration."— Presentation transcript:
Newton’s Second Law If there is a net force on an object, the object accelerates. Its acceleration is directly proportional to the net force Its acceleration is inversely proportional to the object’s mass Its acceleration is in the same direction as the net force.
“directly proportional” means: If the net force doubles, the acceleration doubles. If the net force triples, the acceleration triples. If the net force is half as much, the acceleration is half as much. Etc.
“inversely proportional” means: If the object’s mass doubles, its acceleration will be half as much. If the object’s mass triples, its acceleration will be one-third as much. If the object’s mass is half as much, its acceleration doubles. Etc.
What is “mass”? Mass measures the inertia of an object. All objects made of matter have inertia - that is, they resist accelerations (Newton’s First Law), but some objects resist more than others. Mass is a scalar quantity. SI unit of mass is the kilogram (kg).
Newton’s Second Law In symbols: a = F net m F net = ma F net m a
Preconceptions There are 2 major preconceptions to address: Mass is not the same as weight. Force is not the same as pressure.
Mass is not Weight Mass is a property of an object that measures how much it resists accelerating. An object is difficult to accelerate because it has mass.
Weight Weight is a force - an interaction between 2 objects involving a push or a pull. One of these objects is typically VERY big - the Earth or the Moon, for instance. Weight is NOT a property of an object.
What does weight depend on? The weight of an object depends on the object’s mass. In fact, an object’s weight is directly proportional to the object’s mass. The weight of an object also depends on the object’s location. In fact, an object’s weight is directly proportional to its free fall acceleration, g at its current location.
Weight of a 1 kg object Since W = mg, the weight of a 1 kg object is: W = (9.8 m/s 2 )(1 kg) = 9.8 N on Earth W = (1.6 m/s 2 )(1 kg) = 1.6 N on the Moon
Mass vs. Weight We typically think that an object is difficult to accelerate because it is heavy (has weight) - but it is heavy because it has mass. So, objects are difficult to accelerate because they have mass.
Force is not Pressure Force determines how much an object will accelerate. Pressure determines how that acceleration will feel.
What does pressure depend on? The pressure exerted on an object depends on: The force exerted on the object. In fact, pressure is directly proportional to force. The area over which the force is applied. In fact, pressure is inversely proportional to area.
Pressure Units A force of 1 N applied over an area of 1 m 2 exerts a pressure of 1 Pascal. Another common unit of pressure is pounds per square inch (lb/in 2 ).