Intro
This week is about treating constant acceleration as a model instead of as a formula list. The equations are only useful when the physical situation actually fits the assumptions behind them.
Core Lesson
Constant acceleration means the acceleration stays the same over the interval of interest. That usually happens when the net force is effectively constant and the mass stays constant. It does not mean “the problem looked familiar.”
The important question is not which equation to choose first. The important question is whether the motion story, graph, or force situation supports the constant-acceleration model at all. If the forces are changing dramatically, the model may be poor even if a student can still plug numbers into a kinematics equation.
Kinematics equations summarize the model. Graphs, motion maps, and verbal reasoning should tell you whether the model fits before you write a single symbol.
AP Lift
Modern AP Physics asks whether a model applies, not just whether you can use an equation. Strong students can explain why constant acceleration is or is not reasonable in a given scenario and connect that to graphs and force ideas.
Must-Master Objectives
- Describe the physical conditions that support a constant-acceleration model.
- Connect constant acceleration to nearly constant net force.
- Interpret constant-acceleration motion in words and graphs.
- Explain why using an equation without checking the model can fail.
Problem Set Prompts
- A car speeds up uniformly from rest. What does “uniformly” tell you about the acceleration?
- A ball is thrown straight up after leaving a hand. During the flight, is acceleration constant? Why?
- Sketch a velocity-time graph for an object with constant positive acceleration starting from negative velocity.
- A student wants to use constant-acceleration equations for a rocket whose thrust changes over time. What should they question first?
- If the net force doubles while mass stays fixed, what happens to acceleration?
- Describe a situation where velocity is zero at an instant but acceleration is not zero.
- Explain why a curved position-time graph can still represent constant acceleration.
- Stretch: Write a motion story that has constant acceleration but changing velocity sign.
- Stretch: Give one real-world case where constant acceleration is a useful approximation and one where it is not.
Reflection Prompt
- Do you check whether a model applies before choosing equations, or do you still feel the urge to calculate first?
- What clue helps you most when deciding whether acceleration can be treated as constant?