Book page Syllabus 1.26 and 1.28

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1 Book page Syllabus 1.26 and /15/16 cgrahamphysics

2 Starter Stand with back against the wall, heels touching the wall. 2. Without moving your feet, touch your toes. 3. Explain what is happening. 1/15/16 cgrahamphysics

3 Aim Know what the center of gravity is Understand how stability depends on the center of gravity Understand how forces on a beam act 1/15/16 cgrahamphysics

4 Starter The centre of gravity is where the weight of an object acts through. If the centre of gravity moves outside the base of the object it will fall over. When your centre of gravity moves out from your feet you fall over. 1/15/16 cgrahamphysics

5 What do these images have in common?

6 What is a centre of mass? Can you balance a ruler on the flat end of a pencil? If so, where do you put the pencil to make it balance? The ruler is stationary when the weights of its entire length balance out. This is equivalent to one weight force acting through the centre of the ruler. The point at which all the mass of an object seems to be concentrated is called its centre of mass.

Together, these forces act like a single force pulling at just one point. This single force is your weight.

7 Most people would find it impossible to walk along such a narrow beam, let alone perform hand-stands and somersaults on it. The secret lies in how you position your weight. Every particle in your body has a small gravitational force acting on it. Together, these forces act like a single force pulling at just one point. This single force is your weight. The point is called your centre of gravity or centre of mass. Keep this point over the beam and you stay on. Move either side of the beam, and your weight produces a turning effect which tips you off.

8 The weight is acting straight down from the centre of mass. If the line of action down from the c.o.m. is outside the contact area, the gymnast will fall. What will happen?

9 Centre of Gravity Circus Sketch and number each object and put a cross where you think the Centre of Gravity is. 1/15/16 cgrahamphysics

Stability If something wont topple over, its

back. As long as the line of action stays above

This racing car is even more stable than the

It has a lower centre of mass and a wider base.

10 Stability If something wont topple over, its position is stable. How stable? The truck is in a stable position. If it starts to tip, its weight will pull it back. As long as the line of action stays above its base, it won t topple over. This racing car is even more stable than the truck. It has a lower centre of mass and a wider base. It could be tipped over further before it started to topple. Clever stunt driving, but it has put the car in a potentially unstable position. Tip it any further and the line of action will move outside the base. The car will then topple over.

11 Lines of equilibrium Like the vehicles, the shapes below are all in a state of balance or equilibrium. Why are the shapes stable? Draw in the lines of symmetry for these regular shapes The centre of mass of a regular shape is where the lines of symmetry cross. Therefore their line of action in the cases above, is inside the base each time. The shapes are in a state of balance or equilibrium.

Lines of equilibrium This is fine if you have a regular shape. But how would you find the centre of mass for an irregular shape? You cannot draw lines of symmetry onto these shapes.

12 Lines of equilibrium This is fine if you have a regular shape. But how would you find the centre of mass for an irregular shape? You cannot draw lines of symmetry onto these shapes. Think of an experiment you could perform to find out where the centre of mass is located on each shape. Equipment: Card Scissors Retort stand Boss and clamp Plumb line Discuss with your partner how you might use the equipment to find the centre of mass of each shape.

13 Lines of equilibrium If card is hung from a thread, the centre of mass is always below, in line with the thread. Suspend the card, attach the plumb line. Using a pencil, draw a line along the plumb line. Repeat the last step from a different point. Centre of mass is where the lines cross.

14 Forces on a beam A boy is standing on a beam across a river The beam is not moving à it is balanced, upward and downward force are equal Boy s weight = 400N 1/15/16 cgrahamphysics 200N 200N

15 Change of forces If the boy moves left to one end of the beam, the force distribution changes All the upward force will now be at the left end of the beam 400N 400N 1/15/16 cgrahamphysics

16 Moving again 400N 300N 100N 1/15/16 cgrahamphysics

17 Understanding centre of mass

18 List one thing you already knew, two things you still want to ask and the three most important things that you have learned today.

Question: Bicycles. Vehicle s Static Stability, Part 1. Observations About Bicycles. Vehicle s Static Stability, Part 2

Question: Bicycles. Vehicle s Static Stability, Part 1. Observations About Bicycles. Vehicle s Static Stability, Part 2 Bicycles 1 Bicycles 2 Question: Bicycles How would raising the height of a small pickup truck affect its turning stability? 1. Make it less likely to tip over. 2. Make it more likely to tip over. 3. Have