Newton's Three Laws of Motion — Practice · Grade 12 Science
Chapter: Forces, Rotation & EquilibriumWhat this practice covers
By the end of Newton's Three Laws of Motion you should be able to:
- I can state Newton's three laws of motion and explain inertia as an object's resistance to changes in motion.
- I can apply Newton's second law (F = ma) to solve for net force, mass, or acceleration.
- I can identify action-reaction force pairs and explain why the two forces act on different objects.
10 questions, numeric, multiple choice and written answer, each with a worked explanation.
Vocabulary used: inertia, Newton's first law, Newton's second law, Newton's third law, newton (N).
Problem 1
Answer and explanation
9 m/s²
a = ΣF ÷ m = 36 ÷ 4 = 9 m/s².
Problem 2
Answer and explanation
90 N
ΣF = ma = 0.045 × 2,000 = 90 N.
Problem 3
- The book pulling up on the Earth
- The book's inertia
- The table pushing up on the book
- The book pushing down on the table
Answer and explanation
The book pulling up on the Earth
A third-law pair acts on two different objects. If Earth pulls the book down, the reaction is the book pulling Earth up. (The table's push is a separate force, not the reaction to gravity.)
Problem 4
- Greater than its weight
- Zero
- 9,800 N upward
- 1,000 N upward
Answer and explanation
Zero
Constant velocity means zero acceleration, so by ΣF = ma the net force must be zero — the cable tension exactly balances the weight.
Problem 5
Answer and explanation
Before the impact the passenger's head is at rest. When the car and seat are suddenly pushed forward, the head tends to stay where it was because of inertia (Newton's first law), so it lags behind the moving body and appears to snap backward. A headrest supplies the force needed to accelerate the head along with the body, preventing whiplash.
Problem 6
Answer and explanation
5 m/s²
Net force = 40 − 15 = 25 N east. a = ΣF ÷ m = 25 ÷ 5 = 5 m/s² east.
Problem 7
Hints
The reaction to a push acts on the other object, not automatically in this force sum.
Answer and explanation
F_net=ma=6 N in the acceleration direction.
Problem 8
Hints
State what the supplied observations or conditions establish, and separate that from any assumption.
Answer and explanation
Their interaction forces are equal and opposite; acceleration magnitude depends on each mass.
Problem 9
Answer and explanation
No. Different masses can have different accelerations under equal-magnitude forces, and other forces may also act.
Newton’s third law pairs the force B exerts on A with the force A exerts on B. Their magnitudes match and directions oppose, but they act on different bodies. They therefore do not cancel when calculating the acceleration of A alone. In a combined A+B system these mutual forces are internal and cancel in the total external-force account. Identify the agent and receiving body for every arrow before deciding which sum it belongs in.
No. Different masses can have different accelerations under equal-magnitude forces, and other forces may also act.
Response criteria
- Interpret labels, structures, axes, units and directions correctly.
- Explain the physical or environmental relationship or mechanism.
- Respect stated assumptions and distinguish observation from inference.
Problem 10
Answer and explanation
Net=12 N east; a=3 m/s² east. Its third-law partner is the cart’s 18 N force on the pulling agent, not the 6 N resistance on the cart.
Net=12 N east; a=3 m/s² east.
Its third-law partner is the cart’s 18 N force on the pulling agent, not the 6 N resistance on the cart.
Response criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the physical or environmental mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.