Solutions to Force - Worksheet 1

Inthe diagrams that follow:.Ep: force of engine,fe: air resistance,Jr: force due to friction,
jr : force due to gravityJ. : centripetal force and y - velocity.
(a) Coasting with no pressure on the accelerator: When there is no pressure on the
accelerator there are no forward forces. If there were no backward forces then the car
would keep moving forever at the same speed. However there is rolling friction, air
resistance and friction between the moving parts of the car acting to oppose the
motion of the car. Consequently the car will gradually slow down and stop.
v V
/ 1r // / F /r //// /
*J
(b) Pressing on the accelerator: Pressing on the accelerator increases the rate at which
fuel is fed to the cylinders of the car. This in turn increases the rate of rotation of the
wheels and increases static friction. The reaction force to this increases the forward
force on the car allowing it to overcome the forces that retard the motion. If the
accelerator is held in a position where it balances the forces opposing motion, the car
will maintain constant speed. If the accelerator is pressed a little harder the car will
increase its speed.
V
v
//r/ /1 /
1r// / tt
(c) Pressing on the brakes: The hydraulic system transfers pressure to the pads that grip
the discs or drums. The pads exert a frictional force on the discs or drums that is in the
opposite direction to the motion of the wheel. This slows the rate at which the wheels
are rotating. This causes a forward force between the wheels and the road and the
reaction force to this forward force causes the car to slow down and stop. If too much
pressure is applied to the brakes they completely stop the rotation of the wheels. This
causes the wheels to slide (skid) over the road and since sliding (kinetic) friction is
less than static friction the car travels a much greater distance before stopping.
--v -+
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n /r / / ///
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