Newton’s First Law (Inertia)
A student slides a book across a smooth desk and notices it slows down over time.
Question: What unbalanced force is responsible for stopping the book?
Answer: Friction acts opposite the book’s motion, reducing its speed.
Two identical carts receive equal pushes. Cart A carries extra weight.
Question: Which accelerates more and why?
Answer: Cart B—less mass, so greater acceleration for the same force.
When a swimmer pushes water backward, the swimmer moves forward.
Question: What law explains this?
Answer: Newton’s Third Law—equal and opposite reaction.
What happens to weight when you travel to the Moon?
Answer: It decreases because gravity is weaker.
When forces on an object are balanced, what happens?
Answer: Motion stays constant—no change in speed or direction.
A 1 kg ball and a 3 kg ball fall together (no air resistance).
Question: Compare their motion.
Answer: Same acceleration—gravity affects all masses equally.
Two soccer balls are kicked with equal force—one on grass and one on smooth tile.
Question: Predict which travels farther and explain why.
Answer: The ball on tile; less friction means motion continues longer.
A 2 kg object accelerates at 3 m/s².
Question: What is the net force?
Answer: 6 N (F = ma).
Two skaters push off; one is heavier.
Question: Compare their forces and motions.
Answer: Forces are equal/opposite; lighter skater accelerates more.
Planet A has twice the mass of Planet B.
Question: Which has stronger surface gravity?
Answer: Planet A—gravity increases with mass.
An object moves faster on smooth ice than rough concrete.
Question: Why?
Answer: Less friction reduces opposing force on ice.
Two magnets attract each other. Distance between them doubles.
Question: Predict how force changes.
Answer: Force becomes one-fourth as strong.
A hockey puck glides on ice until it hits a rough patch.
Question: How does this demonstrate Newton’s First Law?
Answer: The puck keeps moving until an outside force (friction) acts on it.
| Trial | Mass (kg) | Force (N) | Acceleration (m/s²) |
|:--:|:--:|:--:|:--:|
| 1 | 2 | 4 | 2 |
| 2 | 2 | 8 | ? |
| 3 | 4 | 8 | ? |
Predict the missing accelerations and describe the pattern between force, mass, and acceleration.
Trial Mass (kg) Force (N) Acceleration (m/s²) 1 2 4 2 2 2 8 4 3 4 8 2
Pattern statement: At constant mass, doubling force doubles acceleration. At constant force, doubling mass halves acceleration. (F = ma)
A rocket expels gas downward.
Question: Describe the motion that results and why.
Answer: Rocket moves upward due to equal/opposite reaction to gas thrust.
| Planet | Mass (×10²⁴ kg) | Radius (×10⁶ m) | Gravity (N/kg) |
|:--|:--:|:--:|:--:|
| Mars | 0.64 | 3.4 | 3.7 |
| Earth | 6.0 | 6.4 | 9.8 |
| Jupiter | 19 | 70 | ? |
Estimate Jupiter’s surface gravity relative to Earth’s and explain why it’s larger.
Use the proportional model g ∝ M / R².
What students should say: Jupiter’s surface gravity is greater than Earth’s because (a) its mass is much larger and (b) even though its radius is larger (which reduces g), the mass effect dominates for giant planets.
If estimating with the given placeholders: Have students justify using the ratio gJ/gE≈(MJ/ME)/(RJ/RE)2g_J / g_E ≈ (M_J/M_E) / (R_J/R_E)^2gJ/gE≈(MJ/ME)/(RJ/RE)2 and explain the trade-off (mass increases g; larger radius decreases g). Full credit for a correct comparison with reasoning about M and R², not just a guess.
A box slides down a ramp; friction increases midway.
Question: Describe the effect on its motion.
Answer: It slows—net force and acceleration decrease.
A student tests an electromagnet by adding batteries.
Question: Predict how strength changes and explain why.
Answer: Increases; more current strengthens magnetic field.
| Object | Force Applied (N) | Friction Present? | Resulting Motion |
|:--|:--:|:--:|:--|
| Astronaut | 5 | No | Moves backward slowly |
| Wrench | 5 | No | Moves forward steadily |
Key ideas: In space (no friction), the net external force during the push is an interaction pair: astronaut pushes wrench forward; wrench pushes astronaut backward with equal magnitude, opposite direction.
Acceptable answer: Both experience equal and opposite forces. The wrench moves forward steadily; the astronaut drifts backward. With no opposing forces, each continues that motion after contact ends.
A car engine’s force increases while its load doubles.
Question: Explain how acceleration changes.
Answer: It may stay the same—doubling both force and mass keeps F/m ratio constant.
A student drops a ball that bounces off the floor.
Question: Explain the force interaction during contact.
Answer: Ball pushes floor down; floor pushes ball up with equal/opposite force.
Earth pulls on the Moon with a certain force.
Question: How does the Moon’s force on Earth compare?
Answer: Equal in strength, opposite in direction.
| Trial | Angle of Ramp (°) | Push Force (N) | Cart Speed (m/s) |
|:--:|:--:|:--:|:--:|
| 1 | 0 | 10 | 2.5 |
| 2 | 10 | 10 | 2.0 |
| 3 | 20 | 10 | 1.3 |
Analyze how increasing incline affects motion and explain using net force.
Observed pattern: As the incline increases (angle rises) with the same push force, the cart’s speed decreases.
Why: The component of gravity along the ramp (m g sinθ) increases with angle and opposes the push, so the net force and acceleration are smaller, giving a lower final speed.
A 2 kg car collides with a 4 kg car at rest; both stick together.
Question: Compare the collision forces.
Answer: Equal and opposite; combined motion follows momentum conservation.
A car stops suddenly and passengers move forward.
Question: Explain how inertia and net force interact in this event.
Answer: Inertia keeps passengers moving; the seat belt provides the unbalanced force to stop them.
During a test, a 1 kg ball and a 3 kg ball are pushed equally.
Question: Which travels farther in the same time and why?
Answer: The 1 kg ball; smaller mass allows greater acceleration from equal force.
| Skater | Mass (kg) | Velocity Before (m/s) | Velocity After (m/s) | Direction of Motion |
|:--|:--:|:--:|:--:|:--|
| A | 60 | 0 | 1.2 | Backward |
| B | 40 | 0 | 1.8 | Forward |
Compare momentum change for each skater and explain how this supports Newton’s Third Law.
Take forward as positive.
Skater A: Δp = m·v_after − 0 = 60 kg · (−1.2 m/s) = −72 kg·m/s
Skater B: Δp = 40 kg · (+1.8 m/s) = +72 kg·m/s
Conclusion: Equal in magnitude, opposite in direction (Δp_A = −Δp_B). This supports Newton’s Third Law and conservation of momentum in the system.
Explain how gravity and inertia keep the International Space Station in orbit.
Answer: Gravity pulls it toward Earth while inertia keeps it moving forward, creating curved orbital motion.
Explain how net force determines direction of motion in a tug-of-war.
Answer: Motion occurs toward the side applying the greater unbalanced force.
| Distance (cm) | Paper Clips Lifted |
|:--:|:--:|
| 1 | 12 |
| 2 | 8 |
| 3 | 4 |
| 4 | 2 |
Predict the number at 5 cm and describe the mathematical pattern in field strength with distance.
Table shows: 1 cm→12, 2 cm→8, 3 cm→4, 4 cm→2 clips.
Prediction at 5 cm: ~1 paper clip (continuing the decreasing trend; many students will notice it roughly halves between later steps).
Reasoning: Magnetic field strength decreases with distance (not linearly). A simple model from the data is “increase distance → significantly weaker force,” consistent with an inverse-power relationship.