Forces Part 2 Free Body Diagrams A free

Forces (Part 2)

Free Body Diagrams �A free body diagram is a diagram used to describe the forces acting on an object �It only describes the forces acting on ONE single object, and not other objects connected to it �Forces are represented by arrows. Direction of arrow represents direction of force Length of arrow represents magnitude of force Each arrow must be labelled

Free Body Diagrams �Objects are typically represented either using a silhouette of the object itself, or a simplification thereof. �E. g. free-body diagram of a cow in free fall COW Gravitational Force (Weight)

Types of Component Forces �Forces can be classified into 2 broad categories: contact forces & long range forces. �Contact forces require an object to physically touch something else in order for the force to take effect �Non-contact do not require physical contact for the force to take effect

Types of Component Forces Contact Forces Normal Contact Force Friction Tension Non-Contact Forces Gravitational Force (Weight) Electric Force Magnetic Force Electromagnetic Force

Half-Time �Breaking The Magician’s Code - Levitation Tricks � 1) Levitating Girl 1 (https: //www. youtube. com/watch? v=OAHs. Y_w 4 lc M) � 2) Levitating Girl 2 (https: //www. youtube. com/watch? v=F 64 L 4 Ct. Gshw) � 3) Flying (https: //www. youtube. com/watch? v=O 0 MOVgz. SU_ E) � 4) Walking on Water (https: //www. youtube. com/watch? v=z 0 uk. Aesc. NC 0)

Gravitational Force �Weight is simply the gravitational force exerted by the Earth on an object �Formula: Gravitational Force = mg, m = mass of the object (which the force is exerting on) g = gravitational field strength. This is also identical to the acceleration due to free fall, which is 10 ms-2. �It is advisable to use “mg” instead of “W” or any other symbol to represent weight or gravitational force.

Gravitational Force �To draw gravitational force of a free body diagram, locate the center of gravity of the object (typically the center of the object) �Draw an arrow downwards (towards the center of the Earth) �Label the arrow “mg” mg

Normal Contact Force �Sometimes called “normal reaction force” �The normal contact force is the force objects exert on each other when they press each other �Normal contact force is always a push force �Usually represented by symbol “N” or “n” �“Normal” here refers to “perpendicular” �The direction of normal contact force is always perpendicular to the surface of contact

Normal Contact Force � To draw normal contact force on a free body diagram, locate the surface of contact � Draw a force originating from the middle of the contact surface, going perpendicular and away from the surface � Label the arrow “N” or “n” N N Object on the ground Object on an inclined surface

Tension �Tension is the force exerted by a taut string, rope, spring, etc. pulling on an object �Usually represented by symbol “T” �Tension is always a pull force �To draw tension in a free body diagram, locate the point where the string (for e. g. ) is pulling the object �Draw an arrow originating from that point, in the same direction as the string �Label the arrow “T”

Tension �Consider this scenario: Ball suspended from the ceiling with a string �Free Body Diagram of the ball: T mg

Tension �Consider this scenario: Object being pulled along the ground by a string N �Free Body Diagram: T mg

Friction �Friction is the contact force that opposes of tends to oppose motion between surfaces in contact �There is no symbol for friction. Either spell the word out in full, or use “Ffriction“ �To draw friction in a free body diagram, identify the contact surfaces, and draw an arrow parallel to the surfaces, in the direction which would oppose motion

Friction moving to the right �If an object is moving in one direction, friction acts in the opposite direction �If friction is holding up an object which would otherwise slide away, friction acts in the direction opposing the sliding Friction

Rules for Drawing FBDs �Step 1: draw the object first. Do NOT draw other objects connected to it �Step 2: identify the center of gravity, draw and label the mg of the object �Step 3: draw all other component forces. The length of the arrows should be proportional to the magnitude of the force. �Note: NEVER draw the resultant force on a Free Body Diagram!! If there is a need to indicate the resultant force, draw it outside the FBD.

Example �Draw the FBD of an object which is being pulled by a rope up a rough slope T N mg Friction

Exercise 2 �An elevator weighs 1000 kg and is pulled upwards by a cable. �(a) Draw the free body diagram of the elevator �(b) If the elevator is accelerated upwards at 0. 1 ms-2, calculate the tension in the cable.