newton's first law

Question

Which of the following statements about a force is correct?
A. A force is required to maintain a body moving with constant velocity
B. The direction of motion of an object is always under the action of an unbalanced force.
C. The speed of an object is always changed when an unbalanced force acts on it.
D. The accleration of an object is always in the same direction of the unbalanced force acting on it.

Explain

Answer 1

D is correct.

For A: Everything has a tendency to keep its motion due to inertia, therefore no force is required to maintain a body moving.

For B: The direction of motion can be different from the unbalanced force acting on it. Consider a jumping boy. After he left the ground and on his way moving upwards, he only experiences the gravity, which is pointing downwards. However, he is moving upwards at the moment. The direction of motion of an object is the same as the direction of the velocity only.

For C: When we apply a force that is perpendicular to the direction of the motion of the object, the speed can remain unchanged. One of the examples is the Moon moving around the Earth. The Earth is always applying gravitational force on the Moon. However, we don't find the Moon will move faster and faster.

For D: Consider the equation F = ma, F and a are both vector and are at the different sides of the equation. They should have the same direction because there is no other vector variable in the equation.