Unit CF S6. You have seen that once an object is moving, if absolutely no forces act on it, it will continue to move at a constant speed. However, based on their real-world experiences, many people hold the idea that a continuous constant force is needed in order to maintain a constant speed. Why do you think this is such a common idea? (Hint: Think about what you have learned in this activity about motion at a constant speed, together with the forces that act on an object when you push it in the real world.) Newton's First Law of Motion You have already seen Newton's Second Law of Motion, which tells us how the rate of change of speed of an object is related to the strength of the net force acting on it. Rate of change in speed = Strength of net force Mass of object From this, you can see that if the strength of the net force acting on an object is exactly zero, then the rate of change of its speed will also be zero, regardless of its mass. A rate of change of zero means that the speed will not change, so the object's speed will remain constant. This is the situation when the forces acting on an object are exactly balanced, so the idea you developed in this activity makes sense in terms of Newton's Second Law. However, Newton thought that this idea was so important that he made it a 'Law' on its own, which we call Newton's First Law of Motion. Using our terminology, we can state this law as: "When the forces acting on an object are balanced, its speed (and direction) will remain constant." Hopefully, it is evident that this applies equally to a situation in which no forces act on an object, because that is just the simplest case of a 'balanced' combination of forces. (Just like a single force is the simplest case of an 'unbalanced' combination of forces.) CF-24Unit CF S4. Two students were discussing their ideas about balanced forces: I just can't see how an object with What I can't understand is how an balanced forces acting on it can be object with balanced forces can be moving at a constant speed. Surely, if moving at all. After all, didn't we learn it's moving in any way, the force in the that an object at rest, with balanced direction of motion must be stronger forces acting on it, will stay at rest? than the opposing force. Luisa Dave How would you respond to Dave's and Luisa's concerns? OS3. Use the pictures below to draw three force diagrams for the cart with balanced forces of a fan and friction acting on it (STEP 4 of Exploration #2): one before the initial push, one during the initial push, and one after the push. Explain your diagrams below the pictures. Before the push During the push After the push CF-21