Motion and Force
What is force?
• Any action that can change the state of
motion of an object. (A push or a pull!)
• Has a magnitude and direction
Fundamental forces
• 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 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.
Newtons?
• Force is measured in N.
– Newton is the force required to cause a mass
of 1kg to accelerate 1 m/s/s
• Force is calculated using: F = ma
– Force is equal to mass x acceleration.