Working Through Holt Physics Chapter 7 Mixed Review
Chapter 7 in Holt Physics is momentum and impulse. The mixed review at the end of the chapter is where students usually get hung up because it mixes several problem types together — conservation of momentum, elastic and inelastic collisions, impulse-momentum theorem, and some two-dimensional problems thrown in for fun. I've watched a lot of kids lose points on stuff they actually understood two days earlier because the review section forces you to switch gears mid-problem. The core equations you need are straightforward. Momentum is mass times velocity: p = mv. Impulse equals the change in momentum: J = Ft = p. In a closed system with no external forces, total momentum before equals total momentum after. That's it. The problems get complicated when they stop giving you clean numbers or when they disguise what type of collision is happening.
Holt Physics Chapter 7 Mixed Review Answers
I can't paste the full answer key here — that's the textbook publisher's content and I'm not going to reproduce it. What I can do is walk through the typical problem types and show you how to actually solve them so you don't need to look up answers blindly. Let me start with the method most students miss. When a problem says "two objects collide," don't immediately write down v1f and v2f like you're in a lab. First, determine whether it's elastic or inelastic. The textbook usually tells you — if it says "stick together" or "move as one," it's perfectly inelastic. If it mentions kinetic energy being conserved, it's elastic. If it just says "collide" without specifying, check whether the problem gives you enough information. Sometimes the answer is that it's inelastic by default because they don't give you the coefficient of restitution or final velocities to check KE conservation. Here's a practical example. Problem type: a 2.0 kg cart moving at 3.0 m/s hits a stationary 1.5 kg cart and they stick together. You find the final velocity using conservation of momentum: m1v1 + m2v2 = (m1 + m2)vf. So 2.0 × 3.0 + 1.5 × 0 = 3.5 × vf. That gives vf = 6.0 / 3.5 = 1.71 m/s. Easy enough. Now check kinetic energy. Initial KE is 0.5 × 2.0 × 9.0 = 9.0 J. Final KE is 0.5 × 3.5 × 1.71² = 5.12 J. Energy is lost. That's expected for inelastic — the point is you confirm it mathematically because some questions ask you to calculate the energy lost, and that's usually worth 2-3 points on its own.
The tricky part students trip on is the sign convention. I once spent twenty minutes on a problem getting the wrong answer because I treated a negative velocity as positive in the momentum equation. The problem had one object moving left at 4 m/s and I wrote +4 instead of -4. The answer was off by nearly 8 m/s in the final result. Always define your positive direction at the top of the problem and stick to it. Draw a quick arrow. Write it down. It takes three seconds and saves you from re-doing the whole thing. Another common pitfall is the impulse problems. Students see a force and time and think they're done. But sometimes the question gives you a force that isn't constant — it's a graph. If you're given a force vs. time graph, the impulse is the area under the curve, not F × t with a single force value. I've seen people use the peak force for the whole duration and get answers that are wildly wrong. If the graph is a triangle, area is 0.5 × base × height. If it's a trapezoid, average the parallel sides and multiply by the base. Know your geometry. Two-dimensional momentum problems are where the real filtering happens. These show up in the mixed review and usually account for the hardest questions. The approach is the same conservation principle, but you break it into x and y components. Total px before equals total px after. Total py before equals total py after. Solve each independently. If one object is moving along the x-axis and after the collision they go off at angles, you'll end up with two equations and two unknowns. Set it up as a system and solve. Don't try to use the one-dimensional formula — it doesn't apply here.
Get the Full Details
Here's a specific edge case I ran into recently that the textbook doesn't really prepare you for. A problem where one object is initially at rest and the other bounces backward after collision. You get a negative final velocity for the first object, which surprises a lot of students. They think they made a mistake. The physics is fine — if a light moving object hits a heavier stationary object elastically, it bounces back. The negative sign is correct. I had a student once remove the negative from his final answer because he "felt like it was wrong," and he lost the point. Trust the math unless it violates a physical constraint. For the mixed review specifically, the questions tend to progress from straightforward applications to things that require you to combine concepts. Problems 1 through maybe 8 or 10 will be direct plug-and-chug. After that, they start layering in energy calculations or multi-step reasoning. If you're spending more than five minutes on a single problem in the first half, you're probably overcomplicating it or missing a simple setup step. Flag it and come back. One thing the answer key won't teach you is how to check your work. After solving, do a quick sanity check. Is the final velocity reasonable? Does it have the right direction? If two identical masses collide elastically and one was at rest, the moving one should stop and the stationary one should take off at the original speed. That's a useful benchmark. If your answer doesn't match that kind of expectation, go back and check your signs and your algebra.
The impulse-momentum theorem also shows up in brake and padding problems — things like airbags, crash mats, and helmet padding. The concept is that increasing the time of impact decreases the force. Same change in momentum, longer time, smaller force. These questions often ask you to compare two scenarios. Calculate the force for each and state which is safer. The math is simple division, but the explanation part is where points are won or lost. If you're looking for the actual answer key, the Holt Physics teacher edition has the complete solutions. Your teacher should have access to it. Some schools post answers on their learning management system. If neither is available, work through the problems methodically using the approach above and you'll end up with the right answers anyway. Looking up answers without doing the work first almost never helps — you'll recognize the numbers on a test but won't know how to set up the problem when the numbers change. Keep a sheet of your common formulas nearby while you work through the review. p = mv, J = Ft = p, m1v1i + m2v2i = m1v1f + m2v2f, and for perfectly inelastic collisions m1v1i + m2v2i = (m1 + m2)vf. Know which one applies when. That's really the whole challenge of this chapter.
