Post on 26-Dec-2015
What is force?
• Any action that can change the state of motion of an object.
• Has a magnitude and direction
• Vary in strength. – SN is strongest, holds nuclei of atoms
together, effective over a very short distance– EM force is about 1/100 of the SN force– Gravitational force is much weaker.
• Can act through contact or at a distance . – Contact push, pull– Field gravity, magnetic
Balanced vs Unbalanced Forces
• Net force – combination of all the forces acting on an object
*When there is a net force acting upon an object, the object accelerates in the direction of the net force.
- No acceleration occurs if net force is 0!
• Balanced forces (no net force) do not change motion.
• Unbalanced forces (net force > 0) do not cancel completely, objects will accelerate in the direction of the greater force.
• Think of tug-of-war!
Friction
• Force that opposes motion between two surfaces that are in contact.
• Occurs because the surfaces of objects are not perfectly smooth.
Types of Friction
• Static friction resists the initiation of sliding motion between two surfaces that are in contact and at rest.
• Kinetic friction opposes the movement of two surfaces that are in contact and are moving over each other.
• Static friction is greater than kinetic friction.
• Different types of kinetic friction – sliding, rolling, fluid, etc.
Friction and Motion
• Friction is necessary for many everyday tasks!
– Think of your typical day!• Tires push against ground to make your car move.• Brakes use friction to stop your car.• Friction between hands & steering wheel.• Friction between pencil and paper allows you to write.• Friction between your feet and floor allow you to push off
when you walk.
Check for understanding...
1. Describe a situation in which unbalanced forces are acting on an object. How is it affected?
2. Identify the type of friction.a. Two students push a box that is at rest.
b. Box pushed by students is now sliding.
c. Students put rollers under box and push it forward.