A force applied to an object moving in a different direction causes the object to accelerate in the direction of the force, altering its speed and/or direction of motion.
Understanding Force and Motion
In physics, a force is an interaction that, when unopposed, will change the motion of an object. This change in motion is called acceleration. As stated in the reference, forces have the ability to accelerate an object. When equal forces push in opposite directions, they balance out, resulting in no net acceleration (and thus no change in motion). However, when an additional force is added in one direction, the object starts to accelerate in that direction.
Acceleration means a change in the object's velocity, which includes both its speed and its direction. This fundamental principle applies whether the object is stationary or already moving.
How Direction Matters
When a force is applied to an object that is already moving, the effect depends on the direction of the force relative to the object's current direction of motion. The force doesn't simply stop the object and then make it move in the new direction; instead, it adds an acceleration vectorially to the object's existing velocity vector.
Think of it like adding velocities: the object's new velocity is a combination of its old velocity and the change in velocity caused by the force (which is acceleration multiplied by the time the force acts).
Impact on Motion
The specific outcome of applying a force to a moving object in a different direction varies:
- Force in the same direction as motion: The object speeds up.
- Force in the opposite direction of motion: The object slows down (decelerates).
- Force perpendicular to the direction of motion: The object's direction changes, causing it to curve. If the force is constant and always perpendicular to the velocity (like gravity on a satellite in orbit or the tension in a string swinging a ball), the object can move in a circle at a constant speed.
- Force at an angle to the direction of motion: Both the object's speed and direction will change. The component of the force parallel to the motion changes the speed, while the component perpendicular to the motion changes the direction.
Examples
Here are a few common examples illustrating how a force in one direction affects an object moving in another:
- Kicking a rolling soccer ball sideways: The ball is rolling forward. Kicking it sideways applies a force perpendicular (or at an angle) to its motion. The ball's path will curve, and its speed might change depending on the angle and strength of the kick.
- Wind pushing on a moving car: A car is moving forward, but a strong crosswind applies a force perpendicular to its motion. This force pushes the car sideways, potentially requiring the driver to steer into the wind to maintain a straight path.
- Gravity on a horizontally thrown ball: When you throw a ball horizontally, it has initial forward velocity. Gravity applies a constant downward force. This downward force causes the ball to accelerate downwards, resulting in the characteristic curved (parabolic) trajectory. The horizontal velocity remains relatively constant (ignoring air resistance), while the vertical velocity increases due to gravity.
Key Takeaway
The effect of a force on a moving object is always to cause acceleration in the direction of the force, which alters the object's existing velocity (speed and direction).
| Direction of Force Relative to Motion | Primary Effect | Resulting Motion Change |
|---|---|---|
| Same Direction | Acceleration (Speed Up) | Increases speed, direction unchanged |
| Opposite Direction | Deceleration (Slow Down) | Decreases speed, direction unchanged |
| Perpendicular Direction | Acceleration (Change Direction) | Changes direction, speed potentially constant |
| At an Angle | Acceleration (Speed & Direction) | Changes both speed and direction |
