Forces Chapter 4

What is a Force? Push or a Pull Must have direction as well as size - vector Can be at a distance or in contact Gravity, friction

4 main Forces in Physics Gravity Electromagnetic Weak nuclear Strong nuclear

Complications of Forces Force is often doing 2 things Example – dragging a desk across the room Pulling over, but also pulling up Want to split force into the two parts

Back to Forces -history Aristotle – natural and violent motion Copernicus – Earth moves! Galileo – Forces are push or pull – friction causes things to slow down, stop

Newton’s Laws Newton comes up with 3 laws that help understand forces and motion 1st Law – “An object in motion will stay in motion; an object at rest will stay at rest unless acted on by an outside force”

Newton’s 1st Law Does this make any sense? Examples? Counter Examples? Sometimes called law of inertia Definition of inertia – tends to do what it is doing Measure of inertia – mass

Equilibrium All forces cancel out Net Force is 0 Needed to apply 1st law (no net outside force) Ex: - skydiver

Friction – What is it? 2 surfaces rubbing together causing motion to be lost - Velcro Tug of war Hard to demonstrate 1st law with friction present Can be from solid, liquid, or gas static vs. sliding friction

Newton’s 2nd Law Here Newton connects the idea of inertia (mass) with those of Force and Acceleration 2nd Law – “The acceleration a force produces is directly proportional to the size of the force and indirectly proportional to the mass of the object” or put in terms of math …F = ma

Newton’s 2nd Law: F=ma Means it is harder to accelerate something with a lot of mass Means something with little mass accelerates with little force

Weight – example of 2nd Law Amount of force that gravity places on an object Is a force Units: Newton = ?

Weight = gravity * mass W = mg; example of F = ma Calculating Weight Weight = gravity * mass W = mg; example of F = ma 9.8(N) = 1(kg)*9.8(m/s2)

2nd Law Problems If you push a 55 kg box across an ice covered pond with a force of 125 Newtons, what is it’s acceleration? A rock has a weight of 225 N. What is its mass? A 65 kg sled has a .20 coefficient of friction. If it is being pulled with a force of 250 N, what is its acceleration?

Newton’s 3rd Law Forces always exist in pairs; if one object exerts a force on a second one, the second one will exert an equal and opposite force on the first one If A acts on B, then B acts on A

Newton’s 3rd Law Examples: Problem: Smart horse Tug-of-war hand on the wall Pulling on a rope Problem: Smart horse Tug-of-war