3.1 forces & free body diagrams

3.1 Forces & Free Body DiagramsRemember that Kinematics is the study of how objects movewithout being concerned about why they move (units 1 & 2)

Dynamics explains why objects move the way they do (unit 3)

What forces are acting on the ice cream cart?

Force

• A push or a pull

• Can make things move or slow them down

• Can distort things

– stretching, compressing, twisting

• Two types:

– action-at-a-distance

– contact

• A vector quantity

Fundamental Forces1. Gravitational force

– Force of attraction between all objects in the universe2. Electromagnetic force

– Caused by electric charges– Holds atoms and molecules together

3. Strong nuclear force– Holds nucleus of an atom together

4. Weak nuclear force– Responsible for elementary particle interactions– Only effective over small distances

Units of Force:

• Measured/calculated in Newtons• 1 N represents the amount of force required

to accelerate a mass of 1 kg at a rate of 1 m/s2.

1 N = 1 kg m s2

Some Common Forces:Type of Force Symbol

Gravity(action-at-a-distance)

FgGravitational pull towards Earth;Force of attraction between any objects that have mass

Normal FNOpposes gravity;Perpendicular to surface that object is in contact with (only on objects in contact with a surface)

Friction FfKeeps surfaces in contact;Parallel to surface that object is in contact with

Kinetic Friction FKOpposes motion of object

Static Friction FSPrevents objects at rest from starting to move

Air Resistance (“drag”) FairFriction on object moving through the air

Tension FTPull on object by string, rope, fibre, or cable

Applied FAAny contact force, in general

Force Due to Gravity

• Where and mass is in kg

• Ex: What is the force of gravity of a person weighing 60kg?

Force DiagramsTo analyze forces and their effects, we use two types of diagrams: System diagram: includes all objects under analysis (eg a drawing of a book

sitting on a desk) Free-body diagram (FBD): shows all forces acting on the object being

analyzed How to draw FBDs:• Object can be represented by a square/rectangle/circle• Forces are represented by vectors (arrows)• Lengths of vectors are not to scale but are proportional to magnitudes of

forces• Indicate which direction is positive

An Example:

Another example:

FAFf

Fg

FN

For practise (right now!):

Try drawing the diagrams for

Solutions

• You must combine all the forces acting on the object as a single combined force.

Ex1: A stationary object exerts a force of gravity of 45N [down] and the floor exerts a normal force of 45 N [up]. What is the next force?

=45N

==

=-45N Therefore there is no net force on the stationary object and therefore it does not move in any direction

Calculating Net Forces

Ex2: Calculate the next force of the object

==

Therefore the object has a net force of 15N [right] which would cause the object to move to the right

Homework: pg 120 # 1-3pg 122 # 1,2,5,7, 15