The motion of an object not moving can change when the net force acting upon it is no longer zero.
Here's a more detailed explanation:
Understanding Net Force
- Forces Acting on Static Objects: Even when an object appears stationary, forces might still be acting on it. For instance, a book resting on a table experiences gravity pulling it down, while the table exerts an equal and opposite force upwards (the normal force).
- Net Force of Zero: When the forces acting on an object are balanced (equal in magnitude and opposite in direction), they cancel each other out. This results in a net force of zero, and the object remains at rest (or continues moving at a constant velocity).
- Net Force and Acceleration: According to Newton's Second Law of Motion (F = ma), force is directly proportional to acceleration. When a net force exists, the object accelerates. This acceleration is the change in velocity over time. Since velocity includes both speed and direction, acceleration means a change in either or both.
How Motion Changes from a State of Rest
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Introducing an Unbalanced Force: The key is introducing a force (or forces) that upsets the balance. This could be:
- Applying a new force: Pushing the book on the table.
- Removing an existing force: Suddenly removing the table from under the book.
- Changing the magnitude or direction of an existing force: Increasing the strength of a wind pushing against a stationary object.
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Resulting Acceleration: Once the net force is no longer zero, the object will begin to accelerate in the direction of the net force. This acceleration is the change in motion from being stationary.
Examples
- A Car at Rest: A car parked on a level surface has gravity acting downwards and the normal force from the road acting upwards. When the engine starts and the driver accelerates, the wheels exert a force against the road, and the road pushes back on the wheels. This unbalanced force causes the car to accelerate forward.
- A Ball on the Ground: A ball sitting still on the ground has gravity and the normal force acting on it. When you kick the ball, you apply an additional force, resulting in a net force that causes the ball to accelerate (move).
Summary
An object's motion changes from a state of rest only when the vector sum of all forces (the net force) acting on it is not equal to zero. The non-zero net force results in acceleration, which is a change in the object's velocity.
