1.5.3 Centre of gravity (3)
Resources |
Revision Questions |
Physics
Login to see all questions
Click on a question to view the answer
1.
A student carried out an experiment to determine the centre of gravity of an irregularly shaped piece of metal. They suspended the metal from a plumb line and drew a vertical line along the thread. They then made a 45° angle with the vertical line and drew another line along the metal's surface. They repeated this process at two other points on the metal. Explain why this method is not ideal for finding the centre of gravity and suggest an improvement.
Why the method is not ideal:
This method is not ideal because the lines drawn at 45° to the vertical will not necessarily intersect at the true centre of gravity. The vertical line of gravity passes through the centre of gravity, but the 45° lines are influenced by the shape of the metal and may not accurately reflect the vertical line of gravity at the centre of gravity. The lines are likely to intersect at a point that is off-centre.
Suggested Improvement:
The student should suspend the metal from a plumb line and repeat the process at more than two points. Ideally, at least three or four different points should be chosen, ensuring they are well-spaced across the metal's surface. The intersection of the vertical lines drawn at these points will give a more accurate determination of the centre of gravity. Using a larger lamina would also improve accuracy.
2.
Question 3
Explain, qualitatively, why a bicycle is more stable when moving than when stationary. Relate your answer to the centre of gravity and the motion of the bicycle.
A bicycle is more stable when moving than when stationary due to the gyroscopic effect and the continuous adjustment of the rider's position to maintain the bicycle's balance. The key principle is the interaction between the bicycle's motion, its centre of gravity, and the base of support (the two wheels).
When stationary, the bicycle's CG is relatively close to the point of contact between the wheels and the ground. This means that even a small imbalance can easily cause the CG to move outside the base of support, leading to a fall.
However, when the bicycle is moving, the gyroscopic effect of the rotating wheels provides a stabilizing force. This force resists changes in the bicycle's orientation. Furthermore, the rider constantly makes small adjustments to their body position to keep the CG aligned with the bicycle's frame and the line of motion. This continuous correction helps to maintain the CG within the base of support. The combination of gyroscopic forces and rider input allows the bicycle to remain balanced even when moving, as the rider can actively counteract any tendency for the CG to drift outside the base of support.
3.
Question 2
A small, rigid block is placed on top of a larger, stable block. Describe, qualitatively, how the stability of the combined system is affected by the position of the centre of gravity of the smaller block relative to the larger block.
The stability of the combined system depends on the combined centre of gravity (CG) of the two blocks and its position relative to the base of support, which is the larger block.
If the CG of the smaller block is directly above the larger block, the combined CG will be very close to the edge of the larger block. This configuration is relatively stable. Any slight movement of the smaller block will have a minimal effect on the overall stability.
However, if the CG of the smaller block is significantly to the side of the larger block, the combined CG will be further away from the edge of the larger block. This makes the system less stable. Even a small nudge to the smaller block could cause the combined CG to move outside the base of support, leading to a loss of stability and potential toppling. The greater the distance of the CG from the base, the less stable the system.