# Forces are usually divided into two types 1

• Slides: 16

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 2. Field forces – Act at a distance through space. The presence of an object effects the space around it, creating a region of space around the object called a field. Example: gravitational field magnetic field around a magnet

CONTACT FORCES � Fn mg Called the ‘normal’ force because it is always normal (perpendicular) to the surface.

CONTACT FORCES � Fn Fn Ffr Fn F mg mg

CONTACT FORCES Friction force Ffr § 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. motion friction

CONTACT FORCES 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 Fg = Fdrag so there is no more

CONTACT FORCES 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.

CONTACT FORCES Tension § the force that the end of the rope exerts on whatever is attached to it. § Direction of the force is along the rope. T 2 T 1 physics T 2 What is the relative force along the two yellow arrows? Why?

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

CONTACT FORCES: THINK PAIR SHARE Type of Force Normal Friction Drag Tension Spring Direction

CONTACT FORCES: THINK PAIR SHARE Type of Force Direction Normal Perpendicular to surface, opposite applied / gravitational forces Friction Opposite motion Drag Opposite motion Tension Along the rope & opposite motion Spring Opposite displacement

FIELD FORCES Field Forces Relative Strength Action Distance Gravitational Force attraction between objects due to their masses 10 -45 Infinite – but decreases with square of distance Electromagnetic Force between charges 10 -2 Infinite – but decreases with square of distance Strong Nuclear Force keeps nucleus together 1 Very short! Weak Nuclear Force arise in certain radioactive processes 10 -8 Very very short!

FIELD FORCES Field Forces Relative Strength Action Distance Gravitational Force attraction between objects due to their masses 10 -45 Infinite – but decreases with square of distance Electromagnetic Force between charges 10 -2 Infinite – but decreases with square of distance Strong Nuclear Force keeps nucleus together 1 Very short! Weak Nuclear Force arise in certain radioactive processes 10 -8 Very very short! At the atomic level – all contact forces are result of repulsive electromagnetic forces – the repulsion of atoms’ electric fields

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

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