Concept Summary Batesville High School Physics

1. Newton's Second Law Concept Summary Batesville High School Physics

2. 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.

3. “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.

4. “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.

5. 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).

6. Newton’s Second Law • In symbols: Fnet a = Fnet m a m Fnet = ma

7. Preconceptions • There are 2 major preconceptions to address: • Mass is not the same as weight. • Force is not the same as pressure.

8. 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.

9. 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.

10. 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.

11. Weight • In symbols: W = mg W g m

12. Weight of a 1 kg object • Since W = mg, the weight of a 1 kg object is: • W = (9.8 m/s2)(1 kg) = 9.8 N on Earth • W = (1.6 m/s2)(1 kg) = 1.6 N on the Moon

13. 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.

14. Force is not Pressure • Force determines how much an object will accelerate. • Pressure determines how that acceleration will feel.

15. 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.

16. Pressure Units • A force of 1 N applied over an area of 1 m2 exerts a pressure of 1 Pascal. • Another common unit of pressure is pounds per square inch (lb/in2).

17. Pressure • In symbols: F F P = A P A

18. The End.