ASVAB · General Science · 7 min read
Motion, Force and Energy
- Differentiate between scalar and vector descriptions of motion, including speed, velocity, and acceleration.
- Apply Newton's three laws of motion and analyze how forces like gravity, friction, and air resistance affect moving bodies.
- Calculate and explain work, power, kinetic energy, potential energy, and the conservation of mechanical energy.
Describing Motion: Speed, Velocity, and Acceleration
When studying physical science for the ASVAB, understanding how objects move begins with the distinction between how far an object travels and where it finishes. Distance is the total length of the path traveled by an object, regardless of twists or turns. In contrast, displacement is the straight-line change in position from start to finish, and it includes direction. For example, if you walk around an oval running track and finish exactly where you started, your total distance traveled reflects the complete length of the path, but your displacement is zero because your position from start to finish has not changed.
How fast an object travels can also be measured in two distinct ways: as a scalar quantity or as a vector quantity. Average speed is a scalar quantity equal to total distance traveled divided by total time taken. Because it is a scalar, speed has magnitude but does not include direction. Velocity is a vector quantity that describes both the speed and the direction of motion. In the metric system, both speed and velocity are measured in meters per second. Knowing an object moves at ten meters per second tells you its speed, but stating that it travels at ten meters per second due north specifies its velocity.
When an object changes its motion, it undergoes acceleration. Acceleration is the rate of change of velocity over time, measured in the metric system in meters per second squared. Because velocity includes both speed and direction, an object accelerates if it speeds up, slows down, or changes its direction of travel. When an object moves at a constant velocity, its speed and direction are unchanged, which means its acceleration is zero. An object slows down when its acceleration points opposite its direction of motion; a negative acceleration does not always mean slowing down, as its effect depends on the object's initial direction of travel.
| Quantity | Type | Metric Unit | Key Definition |
|---|---|---|---|
| Distance | Scalar | Meters | Total length of the path traveled |
| Displacement | Vector | Meters | Straight-line change in position from start to finish, including direction |
| Average Speed | Scalar | Meters per second | Total distance traveled divided by total time taken |
| Velocity | Vector | Meters per second | Speed and direction of motion |
| Acceleration | Vector | Meters per second squared | Rate of change of velocity over time |
Common mistakes
- Assuming that negative acceleration always means an object is slowing down. An object slows down only when its acceleration points opposite its direction of motion; negative acceleration can indicate speeding up in the negative direction.
- Confusing mass with weight. Mass is the constant amount of matter in an object, while weight is the gravitational force acting on that mass, which varies depending on local gravity.
- Thinking action and reaction forces cancel each other out. According to Newton's third law, action and reaction forces always act simultaneously on two different objects, so they do not cancel on a single body.
- Believing that doubling an object's speed only doubles its kinetic energy. Because kinetic energy depends on the square of speed, doubling speed quadruples the kinetic energy.
Key points
- Average speed is a scalar equal to distance divided by time, while velocity is a vector combining speed and direction.
- Acceleration is the rate of change of velocity over time (measured in meters per second squared); it occurs when an object speeds up, slows down, or changes direction.
- Newton's first law defines inertia, which is directly proportional to mass; Newton's second law establishes F = ma; Newton's third law states action-reaction pairs act equally on two different objects simultaneously.
- Mass is constant everywhere, whereas weight is the gravitational force on that mass; in a vacuum, all objects fall with identical acceleration (~9.8 m/s² near Earth's surface).
- Maximum static friction is generally greater than kinetic friction, rolling friction is smaller than sliding friction, and lubricants reduce contact friction.
- Work (force times distance in the force's direction) and energy are measured in joules, while power (the rate of doing work) is measured in watts.
- Mechanical energy is the sum of kinetic and potential energy; the law of conservation of energy states energy cannot be created or destroyed, though friction often converts mechanical energy into thermal energy.
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