Forces are usually divided into two types. 1.Contact forces occur because of physical contact between objects. Examples: pushing open a door pulling on a rope pulling on a rope 2.Field forces – Act at a distance through space. The presence of an object effects the space around it, creating a presence of an object effects the space around it, creating a region of space around the object called a field. region of space around the object called a field. Example: gravitational field magnetic field around a magnet magnetic field around a magnet

mg FnFnFnFn Called the ‘normal’ force because it is always normal (perpendicular) to the surface.

mg FnFnFnFn mg F FnFnFnFn F fr F fr FnFnFnFn

Friction force F fr motionfriction  Friction is a force that is created whenever two surfaces move or try to move across each other.  Friction always opposes the motion or attempted motion of one surface across another surface.  Friction is dependent on the texture/roughness of both surfaces..  Friction is also dependent on the force which presses the surfaces together.

Air resistance / Drag  When an object moves through air or any other fluid, the fluid exerts a friction-like force on the moving object. The force is called drag.  Drag depends upon the speed of the object, becoming larger as the speed increases. (UNLIKE FRICTION!)  Drag also depends upon the size and the shape of the object and the density and kind of fluid. (UNLIKE FRICTION!) B/c drag increases with speed, object moving through the air reach a terminal velocity – a maximum speed at which F g = F drag so there is no more acceleration.

Air resistance / Drag  When an object moves through air or any other fluid, the fluid exerts a frictionlike force on the moving object. The force is called drag.  Drag depends upon the speed of the object, becoming larger as the speed increases. (UNLIKE FRICTION!)  Drag also depends upon the size and the shape of the object and the density and kind of fluid. (UNLIKE FRICTION!) Without drag, raindrops would fall 340 m/h. With drag, they only fall 17 m/h.

physics Tension  the force that the end of the rope exerts on whatever is attached to it.  Direction of the force is along the rope. T1T1 T2T2 T2T2 What is the relative force along the two yellow arrows? Why?

Spring Force  Force due to the elasticity of a material  Depends on the elasticity of the spring  Direction is opposite displacement

Type of ForceDirection Normal Friction Drag Tension Spring

Type of ForceDirection NormalPerpendicular to surface, opposite applied / gravitational forces FrictionOpposite motion DragOpposite motion TensionAlong the rope & opposite motion SpringOpposite displacement

Field Forces Relative Strength Action Distance Gravitational Force attraction between objects due to their masses Infinite – but decreases with square of distance Electromagnetic Force between charges Infinite – but decreases with square of distance Strong Nuclear Force keeps nucleus together 1 Very short! Weak Nuclear Force arise in certain radioactive processes Very very short!

Field Forces Relative Strength Action Distance Gravitational Force attraction between objects due to their masses Infinite – but decreases with square of distance Electromagnetic Force between charges Infinite – but decreases with square of distance Strong Nuclear Force keeps nucleus together 1 Very short! Weak Nuclear Force arise in certain radioactive processes Very very short! At the atomic level – all contact forces are result of repulsive electromagnetic forces – the repulsion of atoms’ electric fields

1. Draw a free body diagram – label all the forces acting on one object. 2. Add up the forces 3. Apply Newton’s second law: F = ma.

How to draw a force diagram 2. Make a simple sketch of the system – point system 1. Choose ONE body to be isolated dog or the cart? fr dog net 3. Identify forces that act on the system Label them on diagram 4. Find out the net force by adding the force vectors decision: cart 5. Apply Newton’s second law