Final answer:
The correct sentences describing how forces affect motion are A, C, and E. Forces can change both the speed and the direction of an object's motion. Net force is responsible for changes in motion, not simply causing motion. Option A,C,E are correct.
Step-by-step explanation:
The question examines how forces affect the motion of objects. Let's clarify the options provided in the question:
A) This sentence is correct. When two forces act on an object simultaneously, they can combine to change the motion of the object. For example, if two people push a box in opposite directions, the box will move in the direction of the stronger push.
B) This sentence is incorrect. An object can move at a constant velocity without any external force acting on it. This is in accordance with Newton's first law of inertia, which states that an object in motion stays in motion with the same speed and direction unless acted upon by a nonzero net external force.
C) This sentence is correct. A force can decelerate an object, making it go slower, which is a form of acceleration (negative acceleration).
D) This sentence is incorrect. Even an object that is not moving can be affected by forces. For example, a book resting on a table is subject to the downward force of gravity and the upward force of the table (normal force).
E) This sentence is correct. Forces can indeed change the direction of an object's motion. When a car turns a corner, the frictional force from the road changes the direction of the car's motion.
Regarding the statement (b) 'Net force causes change in motion' is correct. To understand why, consider Newton's second law which states that the acceleration of an object is directly proportional to the net force acting upon it. Acceleration is a change in velocity, either in speed or direction, therefore, it is the net force that causes this change in motion.
When applying Newton's second law of motion, we can often neglect internal forces such as those holding a body together (cohesive forces) because these forces are usually balanced within the object and do not contribute to its acceleration. The law primarily deals with external forces which are unbalanced and can change the object's state of motion.