Chapter 4 The Laws of Motion

Classical Mechanics Describes the relationship between the motion of objects in our everyday world and the forces acting on them 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 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). Is the tendency of an object to continue in its original motion (or resist a change in motion). Mass is a measure of inertia. Mass is a measure of inertia. Scalar quantity Scalar quantity SI units are kg SI units are kg

4.2 Newton’s 1 st 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.

1 st 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!) The acceleration of an object is directly proportional to 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 2 nd 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)

Force Units SI Force unit: Newton F = ma = (kg)(m/s 2 ) = = 1 N FYI: 1 dyne = 1 g cm/s 2 100,000 dyne = 1 N

Common Forces W n f T