Tutorial Section 7.2 - The assumptions required for constant momentum You have seen previously that Newton's Second Law can be written in terms of momentum: FNet = Ap At , which implies that if the net force on a system is zero, then the system's momentum is constant. In some problems, you may see a system in which the net external force on a system is not zero during a collision. In physics, one of the key abilities to develop is to know what approximations to make in order to solve problems. This section walks through an example in which you will determine whether it is a reasonable assumption to neglect an external force on a system (and assume momentum is constant even in the presence of a nonzero net force on the system). Two 200-g air carts are on the air track The second cart is initially at rest The first cart is moving at a speed of 0.5 m/s before the carts collide head on . . . . After the collision, the first cart is at rest The collision between the carts lasts 50 milliseconds 1. Initially, let's assume that the surface is frictionless. What is the change in momentum of the first cart, looking just before and just after the collision? d of mateys aril mablenow wall ent to ano rhiw woled older and to xod raise ni ihit notelling ed melts bop exoted (085) benimmstab ed Inso on asvaserodes 2. What is the (net) impulse on the first cart during the collision? 3. Now consider friction. The coefficient of kinetic friction between the carts and the air track is 0.0004. What is the force of kinetic friction on the first cart? 4. How much does the net impulse on the first cart change when friction is included? lol exam 5. By what order of magnitude (how many factors of 10) do these two quantities differ: the net impulse on the first cart during the collision, and the impulse on the first cart due to friction during the collision? 6. Is it reasonable to assume the momentum of the system is constant even though friction acts on the system? Why or why not