Conceptual Physics – Newton’s Laws, Forces, and Motion

A set of 22 question-and-answer flashcards covering Newton’s laws, net force, inertia, air resistance, terminal velocity, and propulsion concepts highlighted in the lecture notes.

If gravity between the Sun and Earth suddenly vanished, what path would Earth follow?

A straight-line path (in the direction of its instantaneous velocity).

Which of Newton’s laws best explains why cars need seat belts and air bags?

Newton’s First Law (law of inertia).

What is the net force on an object that is in equilibrium?

Zero net force.

When you quickly jerk a cart forward with a ball resting in the middle, what hits the ball?

The back of the cart strikes the ball because the ball tends to remain at rest (inertia).

A 10-kg block is pushed with 20 N eastward and 15 N westward. What is the net force?

5 N east.

What is the acceleration of that 10-kg block with a 5 N net force?

0.5 m/s² east (a = F/m).

Which Newton’s law relates force, mass, and acceleration (F = ma)?

Newton’s Second Law.

If a cart’s net force doubles while its mass quadruples, how does its acceleration change?

It becomes one-half the original acceleration.

A 100-kg vehicle accelerates at 1 m/s². What net force acts on it?

100 N (F = ma).

A car’s trailer disconnects while the engine force stays the same. What happens to the car’s acceleration?

The acceleration increases because the mass decreases.

At the very top of its flight, what quantity of a tossed rock is momentarily zero?

Its speed (velocity magnitude); acceleration remains g downward.

Does a falling object at terminal velocity still have acceleration?

No; at terminal velocity the net force and acceleration are zero, but the object still has speed.

Why does a heavier parachutist reach a greater terminal speed than a lighter one with the same chute?

He must fall faster so that air resistance grows large enough to match his greater weight.

A 10-kg falling object meets 10 N of air resistance. How does its acceleration compare with g?

It is less than g because the net force is less than its weight.

When a soccer player kicks a ball, how does the force on the ball compare to the force on the foot?

They are equal in magnitude and opposite in direction (Newton’s Third Law).

A karate chop exerts 3500 N on a board. What force acts on the hand?

3500 N on the hand, opposite in direction.

Earth pulls on the Moon and the Moon pulls on Earth. What does this illustrate?

A pair of equal-magnitude, opposite action–reaction forces acting on two bodies.

At terminal speed, what is the air resistance on a 0.8-N flying squirrel?

0.8 N (equal to its weight).

As a squirrel falls faster and faster through air, what happens to air resistance, net force, and acceleration?

Air resistance increases, net force decreases, and acceleration decreases.

Which ball hits first when air resistance matters: a tennis ball or a bowling ball dropped together?

The bowling ball, because air resistance is relatively smaller compared with its weight.

What force directly propels a motor scooter forward on a road?

The forward push of the road on the tires (reaction to the tires pushing backward on the road).

What force actually propels a rocket in space?

The force exerted by the rocket’s exhaust gases pushing downward/outward (action–reaction).