Balanced forces and motion

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Balanced forces and motion
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• Remember that for an object if the resultant
  force is called F, the mass m and the
  acceleration a, then
• F m a
• If there is no resultant force then there is no
  acceleration.
• An object experiencing no net force will continue
  moving at a constant velocity (and remain
  stationary if it was stationary).

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• When a car is stationary there is no resultant
  force.
• The weight of the car is balanced by the upward
  force from the ground. There are no forces acting
  in the direction of the road.
• When the driver wants to move he depresses the
  accelerator pedal which increases the force the
  engine applies to the wheels.
• This produces a resultant force in the forward
  direction and the car accelerates.

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• If the driver continues to hold the accelerator
  pedal at the same point, the car will continue to
  accelerate.
• However, as the car speeds up there are resistive
  forces acting in opposition to the cars motion.
• These are frictional forces from the tyres, air
  resistance and forces of friction in the engine.
  All these forces will be opposing the forward
  forces.
• At a certain speed all these resistive forces
  will be equal to the force from the engine and
  all the forces will again be balanced.
• There will be no net force on the car and the
  acceleration will be zero.
• The car will continue at a steady speed.

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• If the driver comes to a red light he applies the
  brakes. This produces a braking force in a
  direction opposite to the cars motion. In
  applying the brakes he has also had to lift his
  foot from the accelerator pedal so the driving
  force has also been removed.
• This means that the net force is now negative (or
  in the opposite direction to the motion). This
  produces a negative acceleration (or a
  deceleration) which slows the car down.