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
Sliding Friction – force acts at contact between surfaces. Direction of friction opposes velocity or attempted velocity.
Ff = Fn is constant
Which way is friction?
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.
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.