1 of 32 2 of 32 Balanced and unbalanced forces When forces in opposite directions have the same value, they cancel each other out. The forces are balanced. The object is either stationary or moves at a constant speed. If the forces on an object are not balanced, there is a resultant force, and the object accelerates in the direction of this force. 3 of 32 Turning effects from balanced forces 4 of 32 Examples of turning effects These are some examples of everyday turning effects: Can you think of any more? 5 of 32 6 of 32 What is a moment? A moment矩 is a turning effect caused by a force. The size of the turning effect caused by a force depends on two factors. What are they? The bigger the force, the larger the turning effect. The larger the perpendicular 垂 直distance from the pivot 中心点 to the point where the force is applied, the larger the turning effect. 7 of 32 The moment equation The size of a moment obeys the moment equation: moment (Nm) = force (N) × perpendicular distance from pivot (m) In this example, the forces are constant, but the larger the perpendicular distance, the faster the roundabout accelerates: force perpendicular distance 8 of 32 Perpendicular distances 9 of 32 Clockwise and anticlockwise moments When Mark and Tim lift their feet off the ground, what will happen to the see-saw? Tim Mark The force of gravity creates a turning effect on each person. On the left-hand side, it creates an anticlockwise moment. On the right-hand side, it creates a clockwise moment. The clockwise moment is larger than the anticlockwise moment, so the see-saw will turn in a clockwise direction. 10 of 32 The principle of moments If an object is balanced about a pivot, then we can say that: sum of anticlockwise moments = sum of clockwise moments This is the principle of moments. So, the forces on an object must obey two conditions to keep the object completely stationary: 1. The forces must be balanced. 2. The moments must be balanced. 11 of 32 Using the principle of moments 12 of 32 13 of 32 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 moments from 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. 14 of 32 Forces and centres of mass How can the idea of centre of mass help us to understand this statement? “When the forces on an object are balanced, it is either stationary or moves at a constant speed.” What does ‘stationary’ mean if an object is turning? When forces on an object are balanced, its centre of mass is stationary, or moves at a constant speed. 15 of 32 Centre of mass or centre of gravity? Imagine that the gravitational field on the left of this picture suddenly becomes much stronger than the gravitational field on the right. Would the see-saw still balance? The see-saw’s centre of mass is still in the centre of the picture. But its centre of gravity – the point where all its weight appears to act – has moved to the left. In a uniform gravitational field, such as on the Earth’s surface, centre of gravity is equivalent to centre of mass. 16 of 32 Finding a centre of mass 17 of 32 Finding a centre of mass 18 of 32 Understanding centres of mass 19 of 32 20 of 32 When is an object stable? An object is said to be in stable equilibrium if it returns to its original position when it is disturbed. This happens because its centre of mass moves upwards when pushed, so gravity tends to pull the object back down towards its equilibrium position. An object is in stable equilibrium if its centre of mass is suspended below a pivot, or balanced above a wide base. 21 of 32 C.O.M. When is an object unstable? An object is said to be in unstable equilibrium if it falls or topples when it is disturbed. This happens because its centre of mass moves downwards when pushed, so gravity tends to pull the object further away from its equilibrium position. An object is in unstable equilibrium if its centre of mass is balanced above a pivot point or a narrow base. C.O.M. 22 of 32 Stable or unstable? 23 of 32 Neutral equilibrium An object is said to be in neutral equilibrium if it moves sideways when it is disturbed. This happens because its centre of mass stays at the same height when pushed, so gravity does not tend to pull the object towards or away from its original position. Spheres and cylinders are examples of objects that can be in neutral equilibrium when resting on flat ground. C.O.M. 24 of 32 Recognizing states of equilibrium 25 of 32 Losing equilibrium 26 of 32 Making things more stable Look at the picture to the right. Can you explain why this situation is dangerous, using ideas about centre of mass and stable and unstable equilibrium? How could you make the bus more stable? Think of two solutions. 27 of 32 Understanding stability 28 of 32 29 of 32 Glossary 30 of 32 Anagrams slide 31 of 32 Multiple-choice quiz 32 of 32