Vocabulary
This term describes a push or pull that acts on an object as a result of its interaction with another object.
What is force?
In Newton's Third Law, action and reaction forces ALWAYS act on this many objects.
What is two (different) objects?
A firefly hits a car windshield. Which experiences the greater FORCE?
Neither — they experience equal and opposite forces (Newton's 3rd Law).
In a tug-of-war, Team A pulls with 400 N and Team B pulls with 400 N. What is the tension in the rope?
400 N (tension = the pull of either team, NOT the sum).
A 28-kg skater slows from 2.0 m/s to 1.0 m/s in 0.50 s. Calculate the skater's acceleration.
a = (1.0 − 2.0) / 0.50 = −2.0 m/s²
This is the tendency of an object to resist any change in its state of motion.
What is inertia?
When a rocket expels gas downward, the gas pushes the rocket upward. These two forces are an example of this.
What is an action-reaction pair / Newton's Third Law?
A firefly hits a car windshield. Which experiences the greater ACCELERATION and why?
The firefly — same force but far less mass, so a = F/m is much larger.
Arnold pulls one end of a rope and Suzie pulls the other. Who exerts more force ON THE ROPE?
Neither — they exert equal forces on the rope (Newton's 3rd Law / tension).
Using the skater problem (m = 28 kg, a = 2.0 m/s²), calculate the force the skater exerts on the sled.
F = ma = 28 × 2.0 = 56 N
This diagram shows all the forces acting on a single object as labeled arrows.
What is a free-body diagram?
A fish pushes water backward with its fins. Identify the reaction force.
The water pushes the fish forward.
During a collision, a bowling ball exerts 50 N on a pin. How much force does the pin exert on the ball?
50 N in the opposite direction.
What actually determines who WINS a tug-of-war game?
Friction between each team's feet and the ground — not how hard they pull the rope.
An 18-kg ball hits a wall with 9.0 N. What is the recoil acceleration of the ball?
a = F/m = 9.0 / 18 = 0.5 m/s²
This term describes HOW STRONG a force is — its size or amount, measured in Newtons.
What is magnitude?
When you push on a wall, the wall pushes back on you. True or false: these two forces can cancel each other out.
False — they act on DIFFERENT objects and cannot cancel.
An empty shopping cart collides with a full one. They experience equal forces. Explain why the full cart accelerates less.
Greater mass → less acceleration (a = F/m). Greater inertia resists the change in motion.
In a free-body diagram of the WINNING team in tug-of-war, which force must be drawn larger: the rope tension or the ground friction?
Ground friction must be larger — it exceeds the rope tension, producing a net force in the winning direction.
Write the formula for acceleration using change in velocity and time.
a = Δv / t or a = (v_final − v_initial) / t
Newton's Third Law states that for every action force there is a reaction force that is equal in magnitude but opposite in this.
What is direction?
Earth pulls the Moon toward it with 2×10²⁰ N of gravity. How much force does the Moon exert on Earth?
2×10²⁰ N — equal and opposite (Newton's 3rd Law).
After a bowling ball strikes a pin, the pin flies away rapidly while the ball barely slows. Both experienced equal forces. Explain this difference.
The pin has much less mass, so a = F/m gives it a much larger acceleration than the heavy ball.
A spring scale has a 10 N weight hanging from each end. What does the scale read?
10 N — the scale reads tension at one point, not the sum of both weights.
A 5-kg object accelerates at 3 m/s². What net force is acting on it?
F = ma = 5 × 3 = 15 N