# Forces and velocity P2

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• Forces and Velocity
• Drag
• Most resistive forces are caused by air resistance or drag.
• The most important factor in reducing drag in fluids is by keeping the object streamlined.
• Frictional forces from fluids always increase with speed. So at higher speeds the engine has to work much harder to maintain a steady speed.
• Terminal velocity
• When falling objects first set off, the force of gravity is much more than the frictional force slowing them down so they accelerate. As the speed increases the friction builds up and gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force so wont accelerate any more. It has reached terminal velocity.
• Air resistances causes objects to fall at different speeds,and the terminal velocity of any object is determined by its drag in comparison to its weight. The frictional force depends on its shape and area.
• An example of terminal velocity is parachutes.
• As the parachutist moves faster, the upward force of the air resistance increases until it exactly balances the weight. The resultant force is 0 so no resultant force and terminal velocity is reached.
• Speed and velocity
• Speed is how fast something is moving, velocity is speed with a direction. s = d/t
• Acceleration is the change in velocity over time. a = v-u / t
• Forces and elasticity
• When you apply a force to an object you may cause it to stretch and change in shape.
• Any object that can go back to its original shape after the force has been removed is an elastic object. Work is done to an elastic object to change its shape and the energy is not lost but stored by the object as elastic potential energy.
• The elastic potential energy is then converted to kinetic energy when the force is removed and the object returns to its original shape.
• The extension,e, of a stretched spring is directly proportional to the load or force applied,F. The extension is measured in metres and the force in Newtons.      F = k*e
• For small forces, force and extension are proportional, which is shown as a straight line on a graph.
• There is a maximum force that the elastic object can take and still extend proportionally.
• This is known as the limit of proportionality marked by a curved line on a graph.
• k is the spring constant. It's value depends on the material that you are stretching and is measured in newtons per metre, N/m
• There's a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally.
• The accelerating force acting on all falling objects is gravity and all objects fall at the same rate if it wasn't for air resistance.