11 Key Terms newton, - N, unit of force in the metric system - force needed to push a 1kg object at a speed of 1m/s for a second.mass, in physics, the quantity of matter in a body regardless of its volume or of any forces acting on it.weight, measure of the force of gravity on a body
12 What does gravity have to do with the weight of an object? Weight (W) varies depending upon the location of the body in the earth's gravitational field (or the gravitational field of some other astronomical body).The acceleration of gravity on earth is approximately:9.8 m/s² in SI units and32 ft/s² in US Customary units.To calculate the weight of an object you have to multiply it’s mass times the acceleration of gravity.W = m * gAll objects fall at the same speed.Example: Feather vs. concrete blockAir resistance is the only thing affecting how quickly the objects fall. Without that, they fall at the same speed.
13 The Statue of Liberty has a mass of 225,000 kg. How much does she weigh? To calculate the weight of an object you have to multiply it’s mass times the acceleration of gravity.Write the formula:W = m * gSubstitute known values:W = (225,000 kg) * 9.8 m/s²Present solution with units:W = 2,200,000 NWhat the heck is 2,200,000 N?The Statue of Liberty weighs 2,207,250 Newtons, which is 495,000 lbs pounds!
14 Free-Body DiagramA free-body diagram illustrates the relative magnitude and direction of all forces acting upon an object. The object must be isolated and “free” of its surroundings.lbThis is a free-body diagram of the Statue of Liberty. She is represented by a simple box. The forces acting on her are labeled with a magnitude and the arrow shows direction. Notice the surrounding objects are stripped away and the forces acting on the object are shown.
15 Free-Body Diagram“FW” here represents the force of the weight of the statue.“FN” is the normal force, which represents the force Liberty Island is pushing back up on the statue.FW =495,000 lbNormal: means perpendicular to, (ex. The walls to the floor)The force of the pedestal to the statue is normal to the surface of the ground.FN =495,000 lbThe island has a great resistance to compression. The ground is exerting a force upward on the statue perpendicular, or normal, to the surface.
16 Free-Body Diagram Think of the diagram on an XY plane. If “up” is assumed to be the positive direction, then N is positive and W is negative.(Positive y-direction)+yFW =-495,000 lbWhen forces acting on the object cancel each other out it is in a state of static equilibrium.FN =495,000 lb+x(Positive x-direction)
17 Force/Free Body Diagrams Create a free body diagram (FBD) of the gorilla:GorillaFNFWFree Body Diagram of the Sitting Gorilla (The box represents the gorilla, W = weight of the gorilla, N = Normal force)Sitting Gorilla
18 Force/Free Body Diagrams Draw a FBD of the wooden swing:FT1FT2Where are the forces on the swing?SwingFWFree Body Diagram of the wooden swing (The box represents the wooden swing, W = weight of the swing and the parrot, T represents the ropes that are in tension supporting the weight)Parrot on wooden swing hung by ropes
19 Force/Free Body Diagrams Draw a FBD of bucket the bungee jumper leaped from:FTWhere are the forces on the bucket?bucketFWFree Body Diagram of the bucket (T represents the tensile force of the cable the bucket is suspended from, and W is the weight of the diver and the bucket)Bungee jumping from crane
20 Force/Free Body Diagrams Draw a FBD of the ring at point C:Where are the forces on the ring?ABCDFTCAFTCBFTCDFree Body Diagram of the ring at point C (T represents the force of the cables that are in tension acting on the ring)Traffic Light supported by cables
21 Force/Free Body Diagrams Draw a FBD of the traffic light:FTCDWhere are the forces on the light?ABCDLightFWFree Body Diagram of the traffic light (FTCD represents the force of the cables acting on the light and FW is the weight acting on the light)Traffic Light supported by cables
22 Force/Free Body Diagrams Draw a FBD of the pin at point A:Where are the forces on point A?ABEDCFTABFTACFTAEFTADFree Body Diagram of pin A(If you consider the third dimension, then there is an additional force acting on point A into the paper: The force of the beam that connects the front of the bridge to the back of the bridge.)Pin-Connected Pratt Through Truss Bridge