Chapter 4The Laws of Motion

Classical Mechanics

• Describes the relationship between the motion of objects in our everyday world and the forces acting on them

• Conditions when Classical Mechanics does not apply– very tiny objects (< atomic sizes)– objects moving near the speed of

light

Forces

• Force = a push or pull• Vector quantity• May be contact or field force

Contact and Field Forces

Contact: Touching

Field: gravity, charges, magnets

4.1 The 4 Fundamental Forces of Nature

– 1. Strong nuclear force– 2. Electromagnetic force– 3. Weak nuclear force– 4. Gravity

– (FYI all 4 are field forces)

1. Strong Nuclear Force

• Strongest force• Attractive force that holds the nuclei of atoms

together.

2. Electromagnetic Force

• Ex: electrical charges or magnet poles.• Long-ranged but weak• Attractive or repulsive force between objects

carrying electrical charge

3. Weak Nuclear Force

•Very short range and very weak.•Responsible for radioactive decay.

4. Gravitational Force

• Always attractive, and acts between all matter in the Universe.

• Weak, but very long ranged.• It’s the dominant force in the universe for

shaping galaxies and stars. • The forces on the two masses are equal in size

but opposite in direction.

Newton’s 3 Laws of Motion

Inertia

Is the tendency of an object to continue in its original motion (or resist a change in motion).

Mass is a measure of inertia. Scalar quantity SI units are kg

4.2 Newton’s 1st Law of Motion

• “Law of Inertia” = An object at rest stays at rest and object in motion stays in motion unless acted upon by a net external force.

• Net Force = the sum of all forces on an object• Note: Force is a vector, so net force is calculated

by vector addition!

External force– any force between the object and its environment

*Alternative statement of Newton’s First Law*– When there are no external forces acting on

an object, the acceleration of the object is zero.

1st Law Implications• Space objects will move

forever once set in motion

• Friction and air resistance are the net forces that usually slow objects.

Newton’s Second Law(don’t copy this slide!)

m

Fαa

The acceleration of an object is directly proportionalto the net force acting on it and inversely proportional to its mass.

But we like to write it (see next slide):

4.3 Newton’s 2nd Law of Motion

If ΣF (net force) = 0, then there is NO ACCELERATION.

If a constant force is applied to an object, it will ACCELERATE!

(NOT move at a constant speed)

amF

Force Units

• SI Force unit: Newton

• F = ma = (kg)(m/s2) = = 1 N

FYI:1 dyne = 1 g cm/s2

100,000 dyne = 1 N

2s

mkg

Common Forces

W

nf

T