Concept Summary

1
Newton's Second Law

• Concept Summary
• Batesville High School Physics

2
Newtons 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
  objects 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 objects mass doubles, its acceleration
  will be half as much.
• If the objects mass triples, its acceleration
  will be one-third as much.
• If the objects 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 (Newtons First
  Law), but some objects resist more than others.
• Mass is a scalar quantity.
• SI unit of mass is the kilogram (kg).

6
Newtons 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 objects
  mass.
• In fact, an objects weight is directly
  proportional to the objects mass.
• The weight of an object also depends on the
  objects location.
• In fact, an objects 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
A
P
18
The End.