12.1Forces

Objectives:M 4-1

Describe examples of force and identify appropriate SI units used to measure force.

10M 4-2

Explain how the motion of an object is affected when balanced and unbalanced forces act on it.

7M 4-3

Compare and contrast the four kinds of friction.15

M 4-4Describe how gravity and air resistance affect falling

objects. 8

FORCESFORCE: a push or pull that

acts on an object

Measured in Newtons (N)

= a vector: shows direction and magnitude

Net force: is the overall force acting on an object after all forces have been combined.

Balanced and Unbalanced Forces

Balanced forces: a net force of 0. *No Motion

Unbalanced forces: net force not equal to 0.* Causes motion

Examples of ForcesElectromagnetic Forces (between magnets,

protons/electrons)

Nuclear Forces (strong force) (holds the nucleus of an atom together, nuclear energy, atomic bomb).

Centripetal Forces (changes the direction of an object and causes it to move in a circle)

Friction

Gravity

FrictionA force that opposes the

motion of objects that touch each other

Friction happens at the surface of objects (where they touch)

4 main types of friction:Static, Sliding, Rolling,

and Fluid.

Friction between two surfaces

Static Friction Acts on stationary objects.

Always acts in the direction that is opposite of an applied force

Keeps an object from being moved across a surface

Stops acting on an object once the object is moving

Ex. Moving a piece of furniture on carpet.

Static Friction

Sliding Friction Acts on an object once the object is moving

Goes against the direction in which the object is moving as it slides on a surface Occurs when two objects rub together

Less of a force than static friction

Rolling FrictionForce that acts on rolling (round) objects

Acts on the surface and at the point where the object is touching something else (the ground for example)Allows the object to touch the ground and not slipSlows rolling objects down

Fluid Friction Force that opposes an object’s motion through a

fluid (liquid or gas)

Force increases as the object’s speed through the fluid increases

When fluid friction acts on objects moving through the air, it is called air resistance.

Gravity Force of attraction that acts between

any two masses.

Pulls objects together

The magnitude of the force is proportional to the mass of the objects.

Ex. Gravitational pull between sun and earth

Gravitational pull between satellites and the earth.

Gravity on earthPulls objects towards the earth at a rate of

9.8 m/s2

Gravity acts equally on ALL objects, pulling them at the same rate towards the earth.

Objects fall towards the earth at different rates due to air resistance (fluid friction).

Falling objectsTWO Forces acting on a falling object:

1. Gravity (pulling downward)

2. Air resistance (acts in opposition to gravity, reduces acceleration)

Terminal velocity: when air resistance = force of gravity The object is no longer accelerating. V= a x t speed of a falling object

Puzzle…Myth or fact:

If you drop two basketballs with the same shape but different masses (2 kg and 20kg), they will hit the ground at the same time.

True or False, and why?