Newton's 1st Law
What is Newton's 1st law in your own words?
An object in motion stays in motion and an object at rest stays at rest unless acted on by an unbalanced force.
State Newton's Second Law in words (8th-grade level).
An object's acceleration depends on the net force acting upon it and the object's mass.
State Newton's Third Law in one clear sentence for 8th grade.
For every action there's an equal and opposite reaction.
Fill in: Acceleration is (directly/indirectly) related to net force. The larger the net force, the (greater/lesser) the acceleration.
directly; greater.
True/False: Friction is a force. Answer and justify in one sentence.
True — friction is a contact force.
What is Inertia?
An object's natural tendency to resist change in motion.
Fill in the blanks (Accelerate / Not Accelerate / Zero / Non-Zero) : If forces are balanced, the net force is ____ and the object will ____.
If the forces are balanced, the net force is zero and the object will not accelerate.
The skateboard push-off, pushing your foot backward on the ground makes you move forward. Explain using "action" and "reaction".
The action: foot pushes back on ground. Reaction: ground pushes foot forward, propelling rider.
Fill in: Acceleration is directly/indirectly related to mass. The larger the mass, the greater/lesser the acceleration (for the same force).
indirectly; lesser.
Short answer: In the chair-push activity, an empty chair moved farther than a chair with a person when pushed with the same force. Explain why in terms of mass and acceleration.
Empty chair has less mass so larger acceleration for same force (a = F/m).
Which of these statements is true about inertia? (a) Inertia is a force. (b) Objects with more mass have more inertia. (c) Objects only have inertia when moving. (Answer and explain briefly.)
b) Objects with more mass have more inertia.
It is harder to change the motion of something with more mass.
Multiple choice: If an object is accelerating, it must be experiencing: (a) Zero net force (b) Balanced forces (c) A non-zero net force (d) No friction.
(C) A non-zero net force.
Which is the BEST example of Newton's Third Law? (a) A ball rolls to a stop because of friction. (b) A heavier cart accelerates less than a lighter cart under the same push. (c) A swimmer pushes water backward and moves forward. (d) A book stays still on a desk until something pushes it. Pick the letter and explain why.
(c) Swimmer example — push water backward; water pushes swimmer forward.
Action and a reaction
Short answer: What happens if you apply the same force to two objects, one with less mass and one with more mass? Which accelerates more and why?
The smaller-mass object accelerates more because
a = F/m.
An object is pushed with 20 N and accelerates at 5 m/s². What is its mass? Show the work.
4kg
400: If you push a box across a frictionless floor with no air resistance at 5 mph and let go, what will happen to its speed after 5 minutes? Explain using Newton's First Law.
True/False with brief justification: The larger the net force on an object, the greater its acceleration (if mass is constant). Use either the word inversely proportional or directly proportional.
True. (Larger net force → larger acceleration if mass constant.)
Two ice skaters push off each other and move apart. Describe how Newton's Third Law explains their motions and how mass affects each skater's speed after the push.
Both skaters experience equal and opposite forces; the lighter skater gets larger speed change (conservation of momentum context), because acceleration = force/mass.
Calculate: A 30 kg object accelerates at 50 m/s². What is the force? Show the formula you use.
1500N
A car going around a curve at constant speed is accelerating. Explain why this is true and which Newton's Law relates to net force and change in motion.
Constant speed but changing direction → velocity vector changes → centripetal acceleration toward center of curve; net centripetal force causes this acceleration (Newton's Second Law).
Explain why, in the coin-card-cup trick, the coin falls into the cup when the card is flicked. Use the word "inertia" in your explanation.
Because of inertia, the coin wants to remain at rest, so when the card is flicked. The force of the flick is more than the force of friction; therefore, the coin falls straight into the cup.
For each situation say whether net force is Zero (Z) or Non-Zero (NZ):
Short answer: Give a classroom example (different from skateboard or swimmer) that demonstrates action–reaction pairs and identify both forces.
Easiest is probably jumping in the air. The force applied to the ground is what propels the person in the air.
Calculate: A 10 kg object is pushed with 20 N of force. What is the acceleration? Show the formula and final answer.
2m/s².
A=F/M
Bonus-style problem: A 50 kg skater moving at 8 m/s must stop in 2 s. What force must be applied to stop them? Show work and name which Newton's Law explains why a force is needed to change motion.
Δv = 0 − 8 = −8 m/s; a = Δv/Δt = −8/2 = −4 m/s²
F = ma = 50 × (−4) = −200 N−200 N (magnitude 200 N opposite motion). Newton's First Law explains why a force is required to change motion (inertia); Second Law used for calculation.