Types of forces And Free Body Diagrams

What types of forces do you already know?

Forces are usually divided into two types. Different types of forces Forces are usually divided into two types. Contact forces occur because of physical contact between objects. Examples: pushing open a door pulling on a rope 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

Force in Newtons is a VECTOR. Vector Vs Scalar Force in Newtons is a VECTOR. There is a direction, and positive and negative numbers relate to movements on a plane. (Y+ as up, Y- as down) There is a scalar unit for acceleration = Gs. Each G is one Gravity. We are all under one G right now.

Contact Forces Normal force 𝐅 𝐧 Force that prevents an object from falling through the surface of another body Always acts perpendicular to the surface Always equals the forces applied to the surface (or surface will break!) Fn Called the ‘normal’ force because it is always normal (perpendicular) to the surface. mg

Contact Forces Normal force 𝐅 𝐧 Force that prevents an object from falling through the surface of another body Always acts perpendicular to the surface Always equals the forces applied to the surface (or surface will break!) Ffr Fn mg sin q mg cos mg Fn mg F Fn 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.  Friction caused by microscopic roughness in surfaces and, in ultra-smooth surfaces, by molecular attraction 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!) Friction caused by microscopic roughness in surfaces and, in ultra-smooth surfaces, by molecular attraction 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 acceleration.

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!) Friction caused by microscopic roughness in surfaces and, in ultra-smooth surfaces, by molecular attraction 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. What is the relative force along the two yellow arrows? Why? physics T2 T2 T1

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 Direction Normal Friction Drag Tension Spring

Contact Forces: Think PaIR SHARE Type of Force Direction Normal Perpendicular to surface, opposite applied / gravitational forces Friction Opposite motion Drag 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 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 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 Draw a free body diagram – label all the forces acting on one object. Add up the forces Apply Newton’s second law: F = ma.

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

Weight on the moon Divide your weight by 2.2046 to get your approximate mass in kilograms. Weight = Mass x Acceleration of Gravity On Earth, that’s 9.8m/s² but on the moon it’s a measly 1.62m/s² How much do you weigh on the moon?