AP Physics 1 Bell Ringers: 30 Five-Minute Openers

What makes a physics warm-up work

A physics bell ringer earns its five minutes when it forces a prediction, not a recall. Physics rewards this format better than almost any other subject, because most of the ideas that trip students on an exam are ideas they think they already believe, right up until a prediction proves them wrong: acceleration at the top of a toss, the direction a spinning object flies off in, whether a faster student does more work or just more power.

The routine is simple. Put the prompt on the board or the projector. Give every student thirty seconds to write a one-line answer, no discussion yet. Then reveal the resolution, whether that is a demonstration, a simulation, or just the reasoning written out, and spend the last two minutes on why the wrong answer felt right. Nothing here needs printing, grading, or a calculator; a handful of the kinematics, dynamics, and momentum items below do carry small numbers, and those are flagged.

The 30 prompts are grouped by unit so a warm-up always matches the unit currently in progress, plus a few that reach back to keep an earlier unit alive, which is exactly what the cumulative AP Physics 1 exam rewards. Four skills recur across units and are worth building into a habit early: drawing a free-body diagram before touching an equation, checking a limiting case (what happens at zero, what happens at the extreme), tracking the sign of a quantity rather than just its size, and asking whether mass actually belongs in the answer.

Kinematics openers (Unit 1)

These four run before or during the unit on motion graphs, free fall, and two-dimensional kinematics.

• 1. Reading slope as velocity. Prompt: sketch a position-time graph that rises steeply, flattens out, then falls back toward the axis. Ask which direction the object moves during each segment and where its velocity is exactly zero. A correct answer must give the sign of velocity from the sign of the slope in each segment and mark the flat segment, not the point where the curve crosses the axis, as the instant velocity equals zero.
• 2. Area as displacement, not distance. Prompt: a velocity-time graph shows a cart moving forward at a constant speed for three seconds, then reversing at the same speed for two seconds. Ask for the total displacement. A correct answer must use the signed area under the graph, subtracting the reverse segment rather than adding every area as if it were distance traveled.
• 3. Acceleration at the peak of a toss. Prompt: a ball is thrown straight up. Ask whether its acceleration is zero at the exact top of its path, where its velocity is zero. A correct answer must state that acceleration stays at $g$ downward the entire flight, including at the peak, because velocity and acceleration are independent quantities and only velocity is zero there.
• 4. Dropped versus launched horizontally. Prompt: one ball rolls off a table with some horizontal speed at the same instant a second ball is dropped straight down from the same height. Ask which lands first. A correct answer must state that they land at the same time, because the vertical motion of each ball has the same initial vertical velocity (zero) and the same acceleration ($g$), and horizontal velocity never enters the vertical timing. The projectile launcher lets a class fire both cases and time them, and the projectile motion guide has the full setup for follow-up problems.

Dynamics openers (Unit 2)

Four prompts for Newton's laws, friction, and free-body diagrams, the material tied for the largest share of the multiple-choice exam.

• 5. Does the wall push back? Prompt: you push on a wall and it does not move. Ask whether the wall exerts a force on you, and if so, how large. A correct answer must invoke Newton's third law: the wall pushes back with a force equal in size and opposite in direction to your push, and that reaction force acts on you while your push acts on the wall, so the two never cancel each other out.
• 6. Scale reading in a rising elevator. Prompt: you stand on a bathroom scale in an elevator that starts moving upward from rest. Ask whether the scale reads more than, less than, or the same as your true weight during that first second. A correct answer must state that it reads more, because the net upward force needed to accelerate you upward means the normal force (what the scale reads) has to exceed your weight, $F_N = m(g + a)$.
• 7. What friction actually equals. Prompt: a heavy box sits still while a student pushes it gently and it does not move. Ask how large the friction force on the box is. A correct answer must state that friction exactly equals the applied push, not the maximum possible static friction $\mu_s F_N$, because the box is in equilibrium; the maximum only applies at the instant it is about to slip. The static versus kinetic friction guide is the natural follow-up reading.
• 8. Does a heavier block need a steeper ramp? Prompt: two blocks of different mass sit on the same ramp with the same surface. Ask whether the heavier block needs a steeper angle before it starts to slide. A correct answer must state that mass makes no difference, because the slipping condition $\tan\theta \leq \mu_s$ has no mass term in it once $mg\cos\theta$ is divided out of both sides. The inclined plane simulator lets students tilt a ramp and watch the slip angle stay fixed as they swap the mass, and the free-body diagram builder is worth a return visit any week this unit runs long.

Circular motion and gravitation openers (Unit 2)

Four prompts for the circular motion and gravitation topics near the end of the dynamics unit.

• 9. Where does the ball go when the string breaks? Prompt: a ball on a string swings in a horizontal circle, and the string snaps. Ask which direction the ball travels right after. A correct answer must state that the ball flies off in a straight line, tangent to the circle at that point, because with the string gone there is no net force left to curve its path; it does not fly straight outward along the old radius.
• 10. What actually supplies the centripetal force? Prompt: a car goes around a flat, unbanked curve at constant speed. Ask what force is providing the centripetal force that keeps it turning. A correct answer must name static friction between the tires and the road, directed toward the center of the curve, and should not invoke an outward force pushing the car since no such force exists on the car itself.
• 11. Higher orbit, faster or slower? Prompt: two satellites orbit Earth in circular paths, one at a higher altitude than the other. Ask which one moves faster. A correct answer must state that the lower satellite moves faster, since setting gravitational force equal to the required centripetal force gives an orbital speed that decreases as orbital radius increases.
• 12. Doubling the distance between two masses. Prompt: if the distance between two objects doubles, what happens to the gravitational force between them? A correct answer must state that the force drops to one quarter of its original size, from the inverse-square relationship $F = \dfrac{Gm_1 m_2}{r^2}$. The centripetal force guide and its matching calculator back up either prompt with numbers if a class wants to push further.

Energy openers (Unit 3)

Four prompts for work, energy, and power, the unit that shares top billing with dynamics for exam weight.

• 13. Where did the kinetic energy go? Prompt: a sliding box eventually comes to rest on a rough floor with no one touching it. Ask what happened to its kinetic energy. A correct answer must state that friction did negative work on the box, and by the work-energy theorem that negative work equals the drop in kinetic energy, which left the system as thermal energy in the box and floor rather than disappearing.
• 14. A force that does zero work. Prompt: a person carries a box at a steady height across a flat room at constant velocity. Ask whether gravity does any work on the box during the walk. A correct answer must state that gravity does zero work here, because work depends on $F d \cos\theta$ and the angle between the (vertical) weight and the (horizontal) displacement is 90 degrees, so $\cos 90^\circ = 0$ even though the force is present the whole time.
• 15. Where is a pendulum fastest? Prompt: a pendulum is pulled to the side and released from rest. Ask where along its swing its speed is greatest. A correct answer must name the lowest point of the swing, where all of the gravitational potential energy lost since release has converted into kinetic energy, assuming air resistance and friction at the pivot are small.
• 16. Same work, different power. Prompt: two students each lift an identical box up to the same shelf, but one takes twice as long. Ask who does more work. A correct answer must state that both do the same amount of work, since work depends only on force and displacement, and that the faster student instead develops more power, the rate at which that same work gets done. The work-energy theorem guide and conservation of energy guide cover both ideas with full worked numbers.

Momentum openers (Unit 4)

Four prompts for linear momentum and collisions, the last of them a number-light look at how momentum and kinetic energy scale differently with speed.

• 17. Is kinetic energy conserved when carts stick together? Prompt: two carts collide and stick, moving off as one combined mass. Ask whether kinetic energy is conserved in this collision. A correct answer must state no, kinetic energy is not conserved in a perfectly inelastic collision (some of it converts to heat, sound, and deformation), while momentum is conserved in every collision regardless of type.
• 18. Why softening your hands saves the egg. Prompt: catching a raw egg is much safer if you pull your hands back with it as it lands, instead of holding them still. Ask why. A correct answer must connect this to the impulse-momentum relationship $F\Delta t = \Delta p$: since the egg's momentum change is fixed, stretching out the contact time $\Delta t$ lowers the average force $F$ needed to stop it.
• 19. Run the numbers on a two-cart collision. Prompt: a 2.0 kg cart moving at 3.0 m/s hits a stationary 1.0 kg cart; after the collision the first cart continues at 1.0 m/s in the same direction. Ask for the second cart's velocity. A correct answer must apply conservation of momentum, $2.0(3.0) = 2.0(1.0) + 1.0v_2$, giving $v_2 = 4.0\ \text{m/s}$, and should note that this equation holds no matter how the collision splits up the energy. Run it live on the collision lab and check the answer against the momentum collision calculator.
• 20. Momentum doubles, kinetic energy quadruples. Prompt: if an object's velocity doubles, what happens to its momentum, and separately, what happens to its kinetic energy? A correct answer must state that momentum doubles, since $p = mv$ is linear in velocity, while kinetic energy quadruples, since $KE = \frac{1}{2}mv^2$ depends on the square of velocity. The conservation of momentum guide and what is conserved in a collision are the two guides worth pairing with this unit.

Simple harmonic motion openers (Unit 7)

Four prompts for oscillations, using a mass on a spring as the running example.

• 21. Does a bigger pull change the period? Prompt: a mass on a spring is pulled back twice as far as usual and released. Ask whether the period of oscillation changes. A correct answer must state that the period stays the same, since for ideal simple harmonic motion $T = 2\pi\sqrt{m/k}$ depends only on mass and spring constant, not amplitude; the larger pull only makes the mass move faster at every point, not slower or faster in its timing.
• 22. Where is speed greatest, where is acceleration greatest? Prompt: for a mass oscillating on a spring, ask where along its path speed is largest and where acceleration is largest. A correct answer must place maximum speed at the equilibrium position (zero displacement) and maximum acceleration at the two extremes of displacement, since the restoring force, and therefore acceleration, grows with distance from equilibrium while speed does the opposite.
• 23. Doubling the mass on a spring. Prompt: if the mass on a spring doubles while the spring itself stays the same, what happens to the period? A correct answer must state that the period increases by a factor of $\sqrt{2}$, not by a factor of 2, because $T = 2\pi\sqrt{m/k}$ depends on the square root of mass.
• 24. Where does the energy live during the swing? Prompt: describe what happens to a mass-spring system's energy as it moves from maximum displacement to the equilibrium position. A correct answer must state that elastic potential energy converts into kinetic energy over that stretch, with total mechanical energy staying constant if friction and air resistance are ignored: all potential and no kinetic energy at the extreme, all kinetic and no potential energy at equilibrium. The simple harmonic motion guide has the period derivation and more worked cases.

Torque and rotation openers (Unit 5)

Three prompts for torque, rotational inertia, and angular momentum.

• 25. Why does sitting closer to the pivot balance a seesaw? Prompt: a heavier student sits closer to the pivot of a seesaw while a lighter student sits farther out, and the seesaw balances. Ask why this works. A correct answer must state that torque is force times lever arm, $\tau = Fr$, so a smaller lever arm can balance a larger force as long as the two torques come out equal and opposite; force alone does not decide balance.
• 26. Disk versus hoop down a ramp. Prompt: a solid disk and a ring of the same mass and radius are released together at the top of a ramp and allowed to roll down without slipping. Ask which reaches the bottom first. A correct answer must state that the solid disk arrives first, because its rotational inertia, $I = \frac{1}{2}MR^2$, is smaller than the ring's, $I = MR^2$, so less of the disk's energy needs to go into spinning and more is left over for its speed down the ramp.
• 27. The spinning skater problem. Prompt: a skater spinning with her arms out pulls them in close to her body. Ask what happens to her spin rate. A correct answer must state that she spins faster, because angular momentum $L = I\omega$ is conserved with no outside torque acting on her, and pulling her arms in reduces her rotational inertia $I$, which forces $\omega$ to increase to keep $L$ the same. The torque guide, its calculator, and the rotational kinematics guide cover the numbers behind all three.

Fluids openers (Unit 8)

Three prompts for the final AP Physics 1 unit, on fluid statics and flow.

• 28. Why is a dam thicker at the bottom? Prompt: dams holding back water are built noticeably thicker near the base than near the top. Ask why. A correct answer must state that fluid pressure increases with depth, $P = P_0 + \rho g h$, so the deepest water pushes on the dam with the greatest pressure, and the structure needs the most material where that pressure is highest.
• 29. Half-submerged block. Prompt: a block floats in water with exactly half of its volume below the surface. Ask what this tells you about the block's density compared to water. A correct answer must state that the block's density is half that of water, since a floating object's submerged fraction equals the ratio of its own density to the fluid's density, from balancing buoyant force against weight.
• 30. Why water speeds up in a narrow pipe. Prompt: water flows steadily through a pipe that narrows partway along its length. Ask what happens to the water's speed in the narrow section. A correct answer must state that the speed increases there, from the continuity equation $A_1 v_1 = A_2 v_2$ for an incompressible fluid, which requires a smaller cross-sectional area to be paired with a larger speed to keep the flow rate constant. The AP Physics 1 fluids guide walks through pressure, buoyancy, and continuity with full worked numbers.

Running these day to day

Keep the format doing the work rather than the content:

• Same slot, every day. Students who know the first five minutes of class belong to a warm-up start writing before you finish projecting the prompt.
• Do not grade individual answers. Once a warm-up carries points, students protect the points instead of testing their own thinking, and the useful wrong answer, the one you can actually teach from, disappears.
• Always resolve it before moving on. An unresolved prediction teaches students that the warm-up did not matter. A demonstration, a quick calculation on the board, or thirty seconds of reasoning is enough; the free-body diagram builder and projectile launcher both resolve a prediction in well under a minute.
• Bring earlier units back. The cumulative exam in May tests every unit at once, so a kinematics or dynamics prompt is fair game in April; rotate one older-unit item into the queue every week or two.
• Reuse the ones students get wrong. A prompt that a majority of the class misses the first time is worth repeating a month later once the memory of the first attempt has faded. That second pass is where the misconception actually gets fixed.