Transcript Balanced forces and motion - Tasker Milward Physics Website

IP2.7.3 Balanced forces and motion
Balanced forces and motion
© Oxford University Press 2011
IP2.7.3 Balanced forces and motion
 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).
© Oxford University Press 2011
IP2.7.3 Balanced forces and motion
 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.
© Oxford University Press 2011
IP2.7.3 Balanced forces and motion
 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
car’s 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.
© Oxford University Press 2011
IP2.7.3 Balanced forces and motion
 If the driver comes to a red light he applies the
brakes. This produces a braking force in a
direction opposite to the car’s 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.
© Oxford University Press 2011