Warm-Up!

Do objects stop moving once they are set in motion in space? A.Never B.Always C.No, unless something gets in the way

Which set of arrows do not represent equal and opposite force vectors? A. F 1   F 2 B. F 1   F 2 C. F 1   F 2

Inertia and Newton’s Laws

Isaac Newton 18 th Century Scientist Newton’s 3 Laws of Motion Newton’s Law of Gravitation

Galileo’s Thought Experiment © The Physics Classroom, Tom Henderson

This thought experiment lead to Newton’s First Law. Newton’s Thought Experiment © The Physics Classroom, Tom Henderson

Newton’s 1 st Law of Motion Objects will not change their motion without help – force. INERTIA is this resistance to change in motion MASS is how we measure inertia The boulder example

Balanced Forces Balanced forces result in constant motion Forces can be applied but inertia is overcome only if unbalanced forces exist – Leggo my Eggo.

Constant Motion Motion which is either stationary (0) or not accelerating and in a straight line

Forces and Equilibrium If the net force (  F) on a body is zero, it is in equilibrium. An object in equilibrium may be moving relative to us (dynamic equilibrium). An object in equilibrium may appear to be at rest (static equilibrium).

Newton’s First Law Newton’s 1 st Law is also called the Law of Inertia. A body stays at rest or at constant velocity unless acted upon by an unbalanced force. This law is commonly applied to the horizontal component of velocity, which is assumed not to change during the flight of a projectile.  F = 0

The Walking Man man/moving-man_en.jnlphttp://phet.colorado.edu/sims/moving- man/moving-man_en.jnlp

Sample Problem: Three forces act upon a 3.0 kg body moving at constant velocity. F 1 = (4i – 6j + k) N and F 2 = (i – 2j - 8k) N. Find F 3.

Newton’s Second Law Newton’s 2 nd Law says that a body acted upon by a net unbalanced force will accelerate. The acceleration is proportional to the net force and inversely proportional to the mass. It is in the direction of the net force.  F = ma

Sample Problem: Two forces, F 1 = (4i – 6j + k) N and F 2 = (i – 2j - 8k) N, act upon a body of mass 3.0 kg. No other forces act upon the body at this time. What do you know must be true?

Newton’s Third Law Newton’s 3 rd Law is commonly stated “for every action there exists an equal and opposite reaction”. If A exerts a force F on B, then B exerts a force of -F on A.