Centre Of Mass
Übersicht
Take a metre rule and slide one finger along underneath it until the rule sits level and refuses to tip either way. You have just located a single point, and from that moment on the entire rule behaves, as far as gravity is concerned, as though every gram of it had been squeezed into that one spot. Physics calls it the centre of mass. It is one of the shortest ideas on the whole CORE Physics specification and one of the most useful, because once you can find it you can predict which objects will stand and which will fall over.
This lesson shows you how to pin that point down for any shape. For anything symmetrical you can read it straight off the outline. For a shape with no symmetry at all you hang the object up, let gravity do the work, and mark where a weighted thread falls, twice over. Then you will use the point you have found to explain why a racing car can corner at speeds that would roll a delivery van, why the centre of mass of a ring sits in thin air where there is no metal at all, and why a high jumper can pass over a bar that her own centre of mass slips underneath.
Ziele
- The centre of mass of an object is the point at which the mass of the object may be thought to be concentrated. Students will be expected to be able to describe how to find the centre of mass of a thin lamina with irregular shape.
- If freely suspended, an object will come to rest with its centre of mass directly below the point of suspension.
- The centre of mass of a symmetrical object is along the axis of symmetry. The position of the centre of mass affects the stability of objects.
Mindmap
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- Which statement best describes the centre of mass of an object? A. The point representing the mean position of the matter in the object B. The point representing the mean position of the weight in the object C. The point exactly halfway along the longest side of the object D. The point where the object is thickest Answer: A
- An irregular piece of card is hung on a pin and allowed to swing until it stops. Where is its centre of mass? A. At the centre of the card B. At the pin C. Directly below the pin D. Directly above the pin Answer: C
- Where is the centre of mass of a uniform metal hoop? A. In the metal, at the top of the hoop B. In the metal, at the bottom of the hoop C. At the centre of the circle, where there is no metal D. A hoop has no centre of mass Answer: C
- Which change would make a delivery van more stable? A. Fitting narrower axles so the wheels are closer together B. Stacking the heaviest boxes on the roof rack C. Loading the heaviest boxes onto the floor of the load space D. Raising the body of the van higher above the road Answer: C
- An object standing on a table begins to topple when the board it stands on is tilted. At what moment does it start to fall over? A. As soon as the board is tilted at all B. When the downward line from its centre of mass passes outside its base C. When its centre of mass rises above the top of the object D. When its weight becomes greater than the force from the board Answer: B
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