Download Chapter 2 question 4 - leo physics website

Chapter 2 Question 4
a.
State the three origins of force, giving an example for each of them.
3
marks
The three origins of force are
 Gravitational force
e.g. weight of object on the Earth
 Electromagnetic force
e.g. electrostatic force that attracts dust on TV screen and the
magnetic force which causes the motion of a motor
 Nuclear force
e.g. the force that binds the nucleons of an atom together
3
b. Discuss the cause of friction between two bodies in contact, explaining
clearly the dependence of friction on the normal contact force and area of
contact.
2
marks
Friction is caused by the electrostatic attraction between atoms on the two
surfaces.
0.5
Friction increases linearly with the normal contact force. This is reasonable
because the normal contact force increases the interaction between the
atoms.
0.5
Since the surfaces are never flat but are highly irregular, there are only very
small amount of molecules joined together. The number of joints depends
on the normal contact force but is independent of the area of contact.
0.5
0.5
c. Illustrate how the friction between a block on a rough horizontal plank varies
with the applied force on the object. Hence, discuss the motion of the block if 4
the plank is tilted gradually from its horizontal position.
marks
Consider a block being applied with a horizontal force:
1
As the applied force increases, the friction increases. Before the limiting
value is reached, the friction is equal to the applied force. The block is at
rest and the friction is static.
As soon as the block starts motion, the friction is kinetic, which is usually
smaller than static friction. Kinetic friction depends on the normal contact
force only.
When a block is placed on an inclined plane, initially the friction is large
enough to balance the weight down the plane, i.e.
1
1
When the limiting angle is reached, the block starts sliding down. Since
kinetic friction is less than the limiting static friction i.e.
Thus, the block accelerates downward.
1
d. Discuss the roles play by friction in a car during acceleration, normal braking
and emergency braking with skidding. Discuss the use of anti-lock braking
3
system normally equipped in modern cars.
marks
When a car accelerates, the wheels turn faster so that the tyres exert a
backward force on the ground. Thus causes a static friction pointing
forward. Thus, the static friction acts as a propelling force.
In normal braking, the wheels turn slower. This results in a static friction,
pointing backward, between the tyres and the ground. Thus, the static
friction acts as a retarding force.
In skidding, the wheels are locked. The friction between the tyres and the
ground is kinetic and points backward, retarding the motion.
Since kinetic friction is less than static friction, locking the wheels is
undesirable. Anti-lock braking system is to avoid locking. When the system
senses that the wheels are locked, it releases the brakes and then applies the
brakes again.
1
0.5
0.5
1
e. Discuss the importance of tyre threads in a car. Explain the dangers of using 3
tyres without tyre threads on wet roads.
marks
Tyre threads are useful when the road is wet.
The zigzag channels are used to pump water out behind as the wheel rolls
over ponds of water on the road.
If the tyre has no threads on it, a film of water between the road and the tyre
would build up.
This reduces the friction so that skidding would occur.
1
1
0.5
0.5