- Slides: 16
Rope bridge Rope Bridge Gravity Episode 3: Gravity
Objectives: 2 Gravity • To consider the weight of the bridge platform Rope Bridge • To explore the force of gravity and calculate the weight of a variety of objects.
Engage Rope Bridge Hello again! I have a video clip to show you - the person who took it said this was probably the most dangerous rope bridge in the world! 3 Gravity Link: http: //www. youtube. com/watch? v=8 DPP 3 Ei 9 X_0
Engage Rope Bridge 4 Gravity So! Not a nice bridge to cross! What made it dangerous? It was too long? Or too high? Or. . . what do you think? Why do you think the builders of the bridge would make it that way?
Engage Rope Bridge The lack of slats on the pathway does save building materials and reduce the weight of the bridge. But what do we mean by weight? Gravity 5
Explore Weight is a force. If we think about the bridge the weight of the person in the middle pushes down on the bridge platform. The force the person applies to the bridge is called their weight. 6 Gravity Bridge pushes upwards Rope Bridge Weight pushes downwards
Explore Rope Bridge Gravity is a force which acts between masses. Gravity pulls the person and the Earth closer together. Because the Earth is so big it moves very little but the person is small so they can move more easily. We can detect this force pulling the object towards the Earth as weight. 7
Explore 8 Gravity g is the acceleration due to gravity. It is a measure how hard gravity pulls the person and the Earth together. g is the same for all objects all over the Earth. It can be measured at 9. 8 m/s 2. Since weight is a force it is measured in newtons. Rope Bridge We can calculate the weight of an object with this formula: Weight = mass × acceleration of the gravity. W = m × g.
Explore So weight is different from mass? Rope Bridge Yes. Mass is how much stuff is present in an object and is measured in grammes or kilograms. And its measured with a ‘weighing machine’? Gravity Yes! That is a bit confusing. Weight should really be measured with a spring balance. The unit for weight is really newtons - the unit force. Hmmmm. . I think I understand! 9 SS 1
Explore Rope Bridge Gravity I think you’re getting the idea. Why not check your spring balances and weighing machines work properly. Find out the weights and masses of these objects using both methods and see if they agree. 10
Explain Gravity 11 Rope Bridge How did your experiments go? How close were the results from the two methods?
Elaborate Rope Bridge Gravity OK, so we know that the weight of traffic can damage a bridge and that the pathway can be quite heavy. Calculate the weight of the pathways on these bridges. 12
Elaborate Rope Bridge 23 m Gravity Wooden slats are 10 cm wide and spaced 10 cm apart. Each wooden slat weighs 4. 5 kg 13
Elaborate Rope Bridge 23 m 14 Gravity Wooden slats are 12 cm wide and spaced 5 cm apart. Each wooden slat weighs 5 kg.
Evaluate Rope Bridge Gravity This bridge is similar to the one that will replace the damaged one. It is 27. 5 m long and has 20 cm wide wooden slats placed with a 2 cm gap between them. Each slat has a mass of 8 kg. The support rope is made of steel and can carry 25 000 N. 15
Evaluate Rope Bridge Gravity If 60 people cross the bridge at the same time in the morning to go to work what would be the total weight the cables would have to support? Would the bridge be safe? 16