Chapter 4: Turning effects of forces Flashcards
pivot
a fixed point about which a lever turns; also known as the fulcrum
turning effect
when a force causes an object to rotate or would make the object rotate if there were no resistive forces.
moment
The turning effect of a force about a pivot given by force x perpendicular distance from the pivot
equilibrium
when no net force and no net moment act on a body.
when a beam is balanced…
-the forces must be balanced (no resultant force), upward forces = downward forces.
-the turning effects of the forces on it must be balanced (no resultant turning effect)
moment of the force formula
force x perpendicular distance from the pivot
anticlockwise
turning in the opposite direction from the hands on a clock
clockwise
turning in the same direction as the hands on a clock
principle of moments
when an object is in equilibrium the sun of anticlockwise moments about any point equals the sum of clockwise moments about the same point.
stable
an object that is unlikely to topple over, often because it has a low centre of gravity and a wide base.
unstable
an object that is likely to topple over, often because it has a high centre of gravity and a narrow base.
Center of gravity
All the mass of the object could be located here and the object would behave the same (when ignoring any spin).
Lamina
flat, two-dimensional shape
Equilibrium Description(3)
-the glass is upright. Its weight acts downwards and the contact force of the table acts upwards. The two forces are in line, and the glass is in equilibrium.
-The glass is tilted slightly to the right, and the forces are no longer in line. There is a pivot at the point where the base of the glass is in contact with the table. The line of the glass weight is to the left of this pivot, so it has an anticlockwise moment, which tends to tip the glass back to its upright position.
-The glass is tipped further. Its weight acts to the right of the pivot and has a clockwise moment, which makes the glass tip right over.
The moment of a force is bigger (3)
-The moment of a force is bigger if the force is bigger.
-The moment of a force is bigger if it acts further from the pivot
-The moment of a force is greatest if acts 90 degrees to the object it acts on.
Find the centre of mass of an irregular object (6)
-Use a sharp object or a hole puncture to cut holes in 3 different corners (near the edge) on the lamina. they must far apart.
-Fix hole A on the lamina horizontally making sure it can swing properly.
-Hang the plumbline directly in the front of the lamina, wait for the plum bob to come to a rest and then proceed to mark a cross or dot behind the string.
-Take the string and the lamina off the clamp stand gently and from hole A draw a dotted line along the marked indicator.
-Repeat steps 2, 3, and 4 for holes B and C.
-Conclusion- Where the 3 lines intersect, is the center of mass/ gravity on the lamina.
Principle of Moments experiment
-Put a nail into the first cork and clamp it to the stand followed by the meter ruler that must be suspended horizontally at 50 cm.
-Plant the second cork to ensure the meter ruler is in equilibrium.
-Place the m1 weights d2 centimetres away from the pivot on either side of the pivot (the corresponding weight can be moved until the meter ruler is balanced to or away from the pivot.
-Note the results in the table.
-Repeat the experiment with different weights.
Principle of Moments experiment (precautions)
-Make sure the nail is fitted properly into both the corks to balance the ruler better and tightly.
-The meter ruler is at its centre and balanced.
Principle of Moments experiment (safety precautions)
-Wear safety glasses and a lab coat.
-Clamp the retort stand to the bench with a g clamp to avoid it falling on someone’s feet.
-Place an obstacle such as a stool to keep feet beneath the meter ruler/ mass from crushing someone’s feet.
