Newton’s Second Law Quick Check
Which statement correctly applies to a car that changes its velocity over a
period of time? (1 point)
A nonzero net force is acting on it.
B No net force is acting on it.
C Forces are acting on it.
D The combination of all forces acting on it is zero.
The Correct Answer and Explanation is :
The correct answer is A: A nonzero net force is acting on it.
Explanation:
Newton’s Second Law of Motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The law is commonly expressed by the equation:
[
F = ma
]
where:
- ( F ) is the net force acting on the object,
- ( m ) is the mass of the object,
- ( a ) is the acceleration of the object.
When a car changes its velocity over time, it is experiencing acceleration (since acceleration is defined as the rate of change of velocity). According to Newton’s Second Law, an object can only accelerate if there is a nonzero net force acting on it. The net force is the vector sum of all the individual forces acting on the object, such as the force from the engine (if it’s accelerating), frictional forces, air resistance, and gravitational force.
If the car changes its velocity (either speeding up or slowing down), the net force acting on it is not zero. In this case, the forces acting on the car do not balance each other out, resulting in a net force that causes the car’s velocity to change.
Why the other answers are incorrect:
- B: No net force is acting on it. This statement would be true if the car were moving at a constant velocity (not changing its velocity). In that case, according to Newton’s First Law (the Law of Inertia), the forces acting on the car would be balanced, and no net force would be needed to maintain that constant velocity.
- C: Forces are acting on it. This statement is true, but it does not specifically address the fact that the forces are unbalanced and result in a change in velocity. The correct statement must mention that the net force is nonzero, which causes the change in velocity.
- D: The combination of all forces acting on it is zero. This statement would be true only if the car were moving with constant velocity, in which case the forces would balance out, resulting in no acceleration. If the velocity is changing, the forces are unbalanced, so the net force cannot be zero.