Physics Chapter 4

Chapter 4

4-1 Forces

• Force - any kind of push or pull on an object– Ex. Hammer, wind, gravity, bat

• Measuring force– Spring scale

• Force– Vector quantity

• Magnitude and direction

Chapter 4

Newton’s First Law of Motion

• Every body continues in its state of rest or uniform speed in a straight line unless acted upon by an nonzero net force.

• Inertia• tendency of a body to maintain its state of

rest or uniform motion in a straight line.• 1st Law = Law of Inertia

Chapter 4

4-3 Mass

• mass - quantity of matter• mass - measure of inertia of a body• weight - force of gravity acting on a

body

Chapter 4

4-4 Newton’s 2nd Law of Motion

• Relation between acceleration and force

Newton’s Second Law of Motion • The acceleration of an object is directly

proportional to the net force acting on it and is inversely proportional to its mass.

• The direction of the acceleration is in the direction of the net force acting on the object.

Chapter 4

Net force

• Net force = vector sum of all forces acting on an object

F=ma Force = mass x acceleration

• Force - vector quantity - magnitude and direction

• Fx=max Fy=may Fz=maz

• Unit of force = Newton (N)

Chapter 4

Newton

• One Newton (N) is the force required to impart an acceleration of 1 m/sec2 to a mass of 1 kg. 1 N = 1 kg m/sec2

• Examples pg. 82 Ex. 4-1 & 4-2

Chapter 4

4-5 Newton’s 3rd Law of Motion

• Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first.“to every action there is an equal and opposite reaction”

• Push hand on desk, hand distorted• Rocket acceleration

– gases do not push against ground or air– Rocket exerts strong force on gasses, and expels

them, the gasses exert an equal and opposite force on the rocket

• Walking on ice

Chapter 4

Conceptual Example

• Conceptual example 4-3 pg.85

• FGP force of ground on person

• FGP = -FPG

• Conceptual example 4-4 pg.86

Chapter 4

4-6 Weight

• Weight = mass x acceleration due to gravity

• W=mg W = newtons

m = kg

g = -9.8 m/sec2

Chapter 4

Gravity

• g on moon = 1/6 g on earth

• force of gravity - acts while objects falling, acts always

• Normal force - magnitude and opposite direction as weight acts on same object (not 3rd law force)

• Example 4-5 and 4-6 pg.88

Chapter 4

4-7 Free Body Diagrams