Newton's Laws of Motion: 17 MCQs with answers

A · An object stays at rest or in uniform motion unless a net external force acts.

The first law (law of inertia) states that without a net external force, an object maintains its state of rest or uniform motion.

B · Zero, because its velocity is constant.

Constant velocity means zero acceleration, so by the second law the net force is zero. No forward force is needed to keep it moving.

C · Its mass

Inertia depends on mass: a more massive object resists changes in motion more strongly.

A · $\text{kg}\cdot\text{m/s}^2$

From $F = ma$, one newton is the force giving a $1\,\text{kg}$ mass an acceleration of $1\,\text{m/s}^2$, so $1\,\text{N} = 1\,\text{kg}\cdot\text{m/s}^2$.

A · True

Newton's second law is expressed as $\vec{F}_{net} = m\vec{a}$, where force and acceleration are vectors.

B · $3\,\text{m/s}^2$

Using $a = F_{net}/m = 12/4 = 3\,\text{m/s}^2$.

B · $15\,\text{N}$ to the right

Net force is the vector sum: $20 - 5 = 15\,\text{N}$ to the right. Friction must be included.

B · Cart A, because it has less mass.

For the same net force, acceleration is inversely proportional to mass, so the lighter cart A accelerates more.

D · The water pushes the swimmer forward with an equal and opposite force.

This is Newton's third law: the swimmer's backward push on the water is matched by a forward push of the water on the swimmer.

D · They act on different objects, so each object feels only one of them.

Cancellation requires forces on the same object. Action and reaction act on different bodies, so each body accelerates according to the single force it feels.

B · The first law, because the passenger's inertia keeps them moving forward.

The passenger's body tends to keep its forward motion (inertia) when the car decelerates — a first-law effect, which is why seatbelts are needed.

C · $10\,\text{N}$

Using $F_{net} = ma = 2 \times 5 = 10\,\text{N}$.

B · The book pushing down on the table.

Third-law pairs act on different objects and are the same type of force. The normal force from the table on the book pairs with the book's push on the table, not with gravity.

B · The rocket accelerates because the expelled gas pushes back on it.

The third law does not require an external medium: the rocket pushes gas one way, and the gas pushes the rocket the other way.

A · $3\,\text{m/s}^2$ to the right

Net force is $50 - 20 = 30\,\text{N}$ to the right, so $a = 30/10 = 3\,\text{m/s}^2$ to the right. Using the applied force alone would be a mistake.

A · Student B: the cart feels the horse's pull, while the horse feels the cart's pull, so the cart accelerates.

Third-law pairs act on different objects. The cart responds only to the horse's pull on it, so it can accelerate. The forces are equal in magnitude but never cancel because they are on different bodies.

A · 2.5 m/s² to the left

To find the acceleration, first calculate the net force: $F_{net} = 40\, ext{N} - 15\, ext{N} = 25\, ext{N}$ to the right. Then, use the second law: $a = \frac{F_{net}}{m} = \frac{25\, ext{N}}{10\, ext{kg}} = 2.5\, ext{m/s}^2$ to the right. However, since the left force is greater, the correct net force is…