Types of Forces. Four Main Forces in Universe Gravity F g. Electromagnetic EM Strong Nuclear Strong or Nuclear Weak Nuclear Weak All Forces fall into.

Types of Forces

Four Main Forces in Universe

• Gravity Fg.

• Electromagnetic EM

• Strong Nuclear Strong or Nuclear

• Weak Nuclear Weak

•All Forces fall into one of the above categories.

Common ForcesWeight Fg = mg gravity interaction with mass.

Weight depends on gravity,Mass does not.

What is mass?

Amount of matter in object.

• Normal Force

(reaction force) –

• perpendicular to the surfaces in contact.

• Opposite to weight on horizontal surface.

• Fn = -W

What if you’re leaning on a wall. Where is the normal force?

Where is it now?

Sliding Friction – force acts at contact between surfaces. Direction of friction opposes velocity or attempted velocity.

Ff = Fn is constant

Which way is friction?

Which way is the book moving?

Right

Left

The box is not moving.

• Tension –

• transmitted through string, rope, or wire by forces at each end.

Air Resistance – air friction.

Big reason for bad fuel economy in cars.

Spring or Elastic Force – Force applied by stretched

or compressed spring

described by Hooke’s Law.

Magnetic Force

Electrical

Force is a vector quantity. It has direction.

Acceleration – v – is always in the same direction as the net

force!!

Free Body Diagrams

• Diagrams to show relative magnitude & direction of all vector forces acting on a body.

• Sizes of arrows show magnitude of force.

• Arrowhead shows direction of forces.

What can you say for sure about the object in the free body diagram below?

• 1. It is at rest.• 2. It is in motion to the left.• 3. It is in motion to the right• 4. Not enough information to know.

Balanced & Unbalanced Forces

An unbalanced force is a resultant or net force.

What is the Fnet?What can you say about the motion?

12 N 4 N

Fnet = - 8 N or 8 N left

The object shows Acceleration left.

A bird flies with a constant velocity to the left at a constant altitude. What is the free body diagram?

F bird up

F grav

F bird left F air fric

V = 0 up so no Fnet up. F bird cancels Fgrav. Const v horiz. So no Fnet left. Friction balanced by force of bird left.

Assignment

• Handout “Free Body Diagrams”

Number One

Number Two

Number Three

Number Four