AQA A-level Physics Velocity
This page covers the following topics:
The velocity of an object is its speed in a particular direction. While speed is a scalar only measured by its magnitude, velocity is a vector quantity, since it is described both by its magnitude and direction. Velocity is calculated using the same equation as speed; however, velocity usually refers to a specific time and has a direction.
Velocity is usually measured in m/s or km/h, although other compound units may be used. To convert the value of a speed from m/s to km/h, multiply the value by 3.6. To convert speed from km/h to m/s, divide by 3.6.
An object moving in a circular path at a constant speed is constantly changing direction, meaning that the velocity of the object is changing. Since the velocity of the object is not constant, the object must be accelerating, and thus there must be a resultant force acting on it to cause this acceleration. This force is called centripetal force and it acts towards the centre of the circle at 90° angle to the velocity. This acceleration does not affect the speed, only the direction of the velocity. The centripetal force is the resultant of all forces acting in the direction towards the centre of the circle it is travelling through.
Any object dropped near the surface of the Earth will accelerate downwards due to the force of gravity. As the speed of the object increases, the frictional forces (drag/air resistance) increase too. When the frictional forces increase enough to match the weight of the object, the resultant force on the object will be 0. Since there is no resultant force, the object will no longer be accelerating and will be travelling at a constant velocity called the terminal velocity.
In the case of a skydiver, when the parachute is opened, the air resistance suddenly increases due to the great surface area of the parachute, causing the skydiver to slow down. As the speed of the skydiver decreases, the air resistance force decreases too, until a new terminal velocity is reached when the weight of the skydiver and the air resistance become equal again.
The motion of a falling object that reaches terminal velocity can be modelled by the given velocity-time graph. The beginning of the graph shows the acceleration of the object downwards due to its weight. The slope then starts to decrease as the resistive forces start to increase with increasing speed. Eventually, the graph flattens out as the object reaches its constant terminal velocity.
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