Ch 10.3 Forces Force: action on an object to change state of rest or motion (accelerate) (push or pull on object … “baby”answer) Objects change motion because of forces 2 kinds: – Contact forces are touching, like when you push something (friction, applied force) – Field forces do not touch, but act over an area ( field ) like gravity or magnetism

Net forces Net force: total of all forces acting on object if net force = 0 – forces are balanced – object does not change rest/motion if net force is not 0 – forces are unbalanced – object changes state of rest/motion (accelerates)

Net forces Important distinction: – Not true that something that has balanced forces isn’t moving. It could be moving at a constant velocity (speed and direction) – True that something with balanced forces is not accelerating (changing speed, direction or both) Balanced forces = constant motion Unbalanced forces = acceleration (change in motion)

Balanced and Unbalanced So the reason things move differently, in a scientific sense, is that it’s being acted on by unbalanced forces. Can something be moving when the forces on it are balanced? Yes…but it won’t accelerate. It is moving at a constant speed and direction.

Balanced or unbalanced? Car moving in a straight line, 65 mi/hr Car slowing down Car parked Car turning a corner Car speeding up Car stopped at a stoplight with engine running

Balanced or unbalanced?

The Force of Friction Friction: acts against motion of an object (opposite direction of motion) – static : when objects not in motion (parked car on hill) – kinetic : when things are moving (tires on road around turn) Can be rolling or sliding Air resistance: form of friction with air molecules

Friction and Motion Friction can be good or bad Bad: parts in engine rub together. Decrease by: – lubricant (oil) – replace sliding with rolling (wheels, ball bearings) – smooth surfaces Good: keeps car on road. Increase by adding – more force – rougher surface

Newton’s First Law of Motion An object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an unbalanced force Things don’t change their motion (or rest) unless there are unbalanced forces Also known as the Law of Inertia.

Inertia Inertia : tendency of object to not change motion. It’s a property of matter, like density or color or flammability. Not really a number we measure, like forces. More mass you have, more inertia you have e.g. takes more force to move something heavy because it has more inertia (resists that change) JSM (I.2…7 min) w9uNF4nmE w9uNF4nmE

Newton’s Second Law of Motion The unbalanced force acting on an object equals its mass times its acceleration. F = ma Really what it is saying is a =F/m acceleration is directly proportional to force, inversely proportional to mass.

Newton’s Second Law of Motion Force measured in kg x m/s 2 Also called a Newton (N) Mass and acceleration are opposites (if force stays the same). As one gets bigger, the other gets smaller. JSM (II.1, 8 min) EazLCATeYoY EazLCATeYoY

“Ignore friction/air resistance” A lot of times, we’ll talk about “ignoring friction” or ignoring “air resistance.” This is “ideal world” where things are perfect. It tells us what should happen. When we do experiments, we may not always see the same results, because we are in the real world where there is friction and air resistance.

Newton’s Third Law of Motion For every action force there is an equal and opposite reaction force. The forces are equal and opposite,but the resulting motion may not be (larger mass accelerates less) Called force pairs or action/reaction pairs Force pairs do not act on the same object, and thus don’t always have the same effect!

Newton’s Third Law of Motion Note: arrows same size, but opposite direction and acting on different objects (ground and ball, swimmer and water)

Newton’s Third Law of Motion The equal forces don’t always have the same effect, though. According to the second law, with the same force, you have different masses and thus different accelerations. According to first law, you have different inertias depending on mass, so you will resist the change in motion differently. JSM (III.1…7 min) hhB12QXI hhB12QXI

Newton’s Third Law of Motion Things to remember about Newton’s Third Law force pairs – Forces happen at the same time, not one then the other – Forces happen on 2 different objects, not the same one – Forces are equal in magnitude, but the motions are different – Forces are opposite in direction

Newton’s Third Law of Motion Although these balanced forces are equal and opposite, they are NOT a result of Newton’s Third Law! This is a force pair from Newton’s Third Law. The forces are on different objects!