What Misconceptions Exist About Net Force and Motion?

[Kalie]

The 'net force' in the statements below is the sum of ALL forces acting on the body.

During the collision of a car with a locomotive, the locomotive exerts an equal size force on the car as the car exerts on the locomotive.
False

If an object's speed does not change, no net force is acting on the object.
False, there is a net force of zero

If two objects have the same acceleration, they are under the influence of equal forces.
False, F=ma they can have a different mass and thus a different force

The net force which acts on an object which maintains a constant velocity is zero.
True

If a net force acts on an object, the object's velocity will change.
False, the net force acting on the object can be zero

A truck initially moving at a constant velocity will slow down unless a small net force is applied.
False, the trucks velocity will only change if the net force is not zero


This is what I figured out on my own, but I am told it is incorrect. Could someone tell me what I am doing wrong?

 

[cristo]

The 'net force' in the statements below is the sum of ALL forces acting on the body.

During the collision of a car with a locomotive, the locomotive exerts an equal size force on the car as the car exerts on the locomotive.
False

What's your reasoning for this answer?

If an object's speed does not change, no net force is acting on the object.
False, there is a net force of zero

Well, no net force is equivalent to saying a net force of zero. However, note the use of the word speed in this question. An object can accelerate whilst retaining a constant speed (c.f. circular motion). So, your answer of false is correct, but not for the reason you give.

If two objects have the same acceleration, they are under the influence of equal forces.
False, F=ma they can have a different mass and thus a different force

Correct

The net force which acts on an object which maintains a constant velocity is zero.
True

If a net force acts on an object, the object's velocity will change.
False, the net force acting on the object can be zero

Again, "a net force" means "a non-zero net force"

A truck initially moving at a constant velocity will slow down unless a small net force is applied.
False, the trucks velocity will only change if the net force is not zero

Correct.

This is what I figured out on my own, but I am told it is incorrect. Could someone tell me what I am doing wrong?