Final answer:
Simple machines such as levers and pulleys change the direction or size of a force. Net forces are central to motion, causing changes rather than simply creating motion. Newton's laws of motion explain the relationship between forces and movement of objects.
Step-by-step explanation:
Devices that Change the Direction or Size of a Force
Devices that change the direction or size of a force are commonly known as simple machines. These include the lever, pulley, wheel and axle, inclined plane, screw, and wedge. Each of these devices either increases the magnitude of a force, changes the direction of a force, or both, which can make tasks easier to perform. For example, a lever allows a smaller force to lift a heavier object by increasing the distance over which the force is applied.
Understanding Forces in Motion
Net forces are critical in understanding motion. According to Newton's first law of motion, an object will stay at rest or move at a constant velocity if the net force is zero. If there is a net force, Newton's second law of motion comes into play, indicating a cause of acceleration. For example, when kicking a soccer ball, the force of the kick changes the motion of the ball. Similarly, when a dolphin jumps out of the water, gravity and the force exerted by the dolphin itself affect its motion. A parachutist drifting to Earth is subject to gravity and air resistance, which are external forces.
Net Force and Motion
It's essential to note that statement (b) 'Net force causes change in motion' is correct. A net force does not simply cause motion, but it causes a change in motion, such as starting, stopping, or changing direction. For instance, a car at rest requires a net force to start moving, and once moving at constant velocity in the absence of friction, the net force is zero.