Motion and Force. What is force? Any action that can change the state of motion of an object. Has a magnitude and direction.

Motion and Force

What is force?

• Any action that can change the state of motion of an object.

• 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 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.

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.