Newton developed the concept of momentum or "quantity of motion" of an object. He defined the linear momentum, p, as the product of an object's mass, m, and its velocity, v. Note that in this section, we use the term momentum to refer to linear momentum. Later on when we study rotational motion we will define another sort of momentum, "angular momentum." The SI units for momentum are kilogram-meters per second. Since momentum is the product of the mass of an object, which is a scalar quantity, and its velocity, which is a vector, momentum is also a vector. The direction of the momentum vector is the same as the direction of the velocity. So what is the physical meaning of momentum? Let's consider a car and a truck moving at the same speed. Since the truck is heavier than the car, the truck has greater momentum than the car. Thus it takes the truck longer than the car to stop. Similarly, if two identical cars move at different speeds, it will take longer for the faster car to stop. Again, this is because the faster car has greater momentum. Obviously, a speeding truck is more difficult to stop than a lighter, slower moving car. Let's now calculate the momentum of some objects. The momentum of a 2-kilogram ball moving at a constant velocity of 2 meters per second is 4 kilogram meters per second. What is the momentum of a 1 point 5 kilogram ball going in the opposite direction at a constant speed of 3 meters per second? ...

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