Force Force is a push or pull on an object The object is called the System Force on a system in motion causes change in velocity = acceleration Force is.

Force

• Force is a push or pull on an object

• The object is called the System

• Force on a system in motion causes change in velocity = acceleration

• Force is a vector, it has direction

• External World = everything around the system that exerts a force

Types of Forces

Forces Contact

• When the external world is in contact with the system and exerts a force ex. Hand that pushes, Rope that pulls

Field Forces

• A force exerted on a system without touching the system ex. Gravity, Magnetic force, Interaction between charged particles

Agent

• An agent is the item in the external world that causes the force to act on the system

• ex. The hand that holds

The rope that pulls

The earth

The magnet

Free Body Diagram

• A diagram representing the System, Agent and the Forces acting on the system

• System is represented by a dot• Forces represented by arrows pointing in the

direction of the force, away from system• Label each force with a subscript• Choose direction of +ve usually towards

stronger force

Ft

gF

Ball

String

(force of tension)

(force of gravity)

Positive Direction

Combining Forces

• As forces are vectors they can be added just as vectors

ex.1 Fa + Fb = F net

ex.2 Fa + Fb = Fnet

Nonlinear non perpendicular forces are added by adding x and y components of the force vectors

Newton’s First Law

• An object at rest will remain at rest and an object in motion will remain in motion unless acted on by an external net force

• Also known as the Law of Inertia

• Inertia is the tendency of an object to resist change in motion

• Equilibrium = if the net force on a system is zero the system is in equilibrium speed and direction is unchanged.

Common Types of Forces

• Friction Ff = Contact force opposing motion of two surfaces, parallel to the surface and opposite to the direction of motion

• Normal FN = Contact force exerted by system’s surface perpendicular to and away from surface

• Spring F sp = A restoring force- push or pull of spring exerted on system opposite to displacement

• Tension FT = pull exerted by a rope on a system, away from the system and parallel to the rope

• Thrust FTHRUST = a force that moves a system in the same direction as the acceleration

• Weight F g = a field force on a system due to gravity directed to the center of the earth

Newton’s Second Law

• The acceleration of an object is equal to the net force acting on it divided by the mass of the object a = F/m or F = ma

• The larger the force the greater the acceleration The greater the mass with the same force the lower the acceleration

• Force of 1N = 1kg * 1m/s.s

Newton’s Third Law

• All forces come in pairs equal and opposite in direction

• ex. A ball on a table and the table on the earth. Forces-ball on table / table on ball

-table on earth/ earth on table

- ball on earth/ earth on ball

Net free body diagram

ball

tableearth

F table on ball

F earth on ball

Drag Force and Terminal Velocity

• Drag Force is the force exerted on an object as it moves the a fluid ex. Air and water

• as the speed of the object through the fluid increases so does the drag

• Drag is effected by the fluids viscosity and temperature

• Terminal velocity is when the drag force equals the force due to gravity no acceleration constant velocity app.(60m/s)

Tension

• Is a force exerted by a string or rope on a system

• It is assumed the rope has no mass

• Tension within all points of the rope is equal and opposite to the force exerted by the system’s weight

Example of bucket on rope

FT

Fg =mg

Normal Force

• Is a contact force exerted by a surface on an object. This force is perpendicular to the surface of contact

Fg

N

Gravitational Force

• Gravitational force is the mutual attraction between any two bodies in the universe

• Newton’s Law of Universal Gravitation =every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them

• F g = Gm1m2/r2 where G = the universal gravitational constant = 6.67*10-11N.m2/kg2

Gravity

• As weight = mg then w = GMEm/r2

• Then g = GME/r2

• Therefore the larger the mass of an object the larger the effective gravity it generates

Forces of Friction fs and fk

• An object moving on a surface or passing through a fluid experiences resistance to motion = friction

• F fs = forces of static friction = the force that prevents movement of an object that is being subjected to an external force

• When movement is about to occur fs is at max.

fs and f k

• When a force F exceeds Ffs max then movement will occur and the new friction force is less. This new friction force is called the Force of kinetic friction fk

• When F-fk= positive value there is acceleration• Both fs and fk are proportional to the normal force;

fs=usn and fk=ukn• us is the coefficient of static friction • Uk is the coefficient of kinetic friction

Solving Friction Problems

• Draw a free body diagram making sure to label all forces