Chapter 5 Section 2 - 3 Forces

Forces Force – a push or pull on an object English Units Pound (lb) SI Units Newton (N)

More than one force acting on an object Net Force – the sum of all the forces acting on an object Balanced Force – forces are equal but opposite, causes no change in motion to object Unbalanced Force – forces are unequal, causes a change in motion to object

Affect on Motion A balanced force is applied on a stationary object Object will not move An unbalanced force is applied on a stationary object Object will start to move in the direction of the unbalanced force A balanced force is applied on a moving object Object will not change its motion An unbalanced force is applied on a moving object Object will change its motion in the direction of the unbalanced force

The Most Important Statement in Physics! Unbalanced forces cause acceleration.

Free Body Diagram Used to find net forces on objects Draw a box to represent object Draw arrows to represent direction of force Label arrows with the force Find the net force

Free Body Diagrams 15N 10N 15N – 10N= 5N right

Calculating Net Force Change direction to positives and negative signs

Friction Friction – a force that opposes motion (opposite direction) Source – bumps and grooves in surface of objects Factors that effect friction Roughness of surface Force pushing surfaces together (more force pushes bumps and grooves together)

2 Types of Friction Static Friction – friction on a stationary object when a force is applied (balanced force) Kinetic Friction – friction on a moving object (can be balanced or unbalanced based on the net force between the applied force and friction) Sliding – when one surface slides past another Rolling – when one surface rolls over another Fluid – when particles of a liquid or gas collide with the surface of an object

Ways to Reduce Friction Lubricants (oil, grease, air) Switching from sliding kinetic friction to rolling kinetic friction (add wheels, ball bearings etc…) Make surfaces smoother (polish the surfaces) Reduce the force pushing surfaces together

Ways to Increase Friction Make surface rougher (adding sand to ice) - use a textured surface like a scouring pad to clean pots or gloves for baseball Increase the force pushing the surfaces together