Download Senior Science topics Programme

Senior Secondary Science Programme
Sequel to Newtonian Mechanics
Teachers' Notes
Subject
Physics
Level
S4-S6
Duration:
15 minutes
Key Points
1. Newton’s Third Law of motion and identify action and reaction pair forces.
2. Inertial forces.
Content:
1.
Newton’s Third Law of motion
“To every action, there is an equal but opposite reaction.” This is Newton’s third law of motion.
Different daily examples were used to illustrate the third law:
When swimming, we have to paddle or push water backwards forcefully to move faster. The
body exerts force in the water and it acts backwards. At the same time the reaction force by the
water pushes the body forward. This is more apparent at revering. We have to stretch the leg on
the wall of the pool forcefully to order to reverse. The body applies force to the wall, and the
body is simultaneously acted upon by the reaction force. The body rebounds rapidly because of
the reaction force.
Although action and reaction occur at the same time and in opposite directions, they absolutely
can’t cancel each other as they are acting on two different bodies.
Two ice skaters are standing towards each other. One pushes the second one. According to
Newton’s Law, the second one will also reacts to the first one with a force equal in magnitude
but opposite in direction. As the reaction force acts on a different body and it is not on the body
acted by the action force, so the forces will not cancel each other.
Action and reaction can be applied to movement in daily life. For example, a car can move
forward because of its reaction from the ground when its wheels rotate and push down on the
ground. A boat pushes water backwards with propeller and the reaction from water pushes the
boat to move. Jet plane forcefully push air backwards by jet engine and the reaction of the air
pushes the jet forward.
Action and reaction forces occur at the same time and equal to each other. They should be
identically the same in physical nature. Just like an apple resting steadily on the table. It is due
to the balance of gravity and the upward reaction by table. These two forces are not related to
the third law as they are of different physical nature and acting on the same object. Gravity is
the action by the Earth on the apple, its reaction is the force by the apple on the Earth – the
Earth acts on the apple and the apple acts on the Earth. The two do not need to contact each
other.
2.
Inertial force
You feel a force pressing you into the seat when a vehicle or bus suddenly accelerates; or you
feel a force throwing you forward when it suddenly stops. This force is called inertial force.
But in fact inertia force doesn’t exist, so it is also known as fictitious force.
Inertial force can only be experienced in an accelerated environment, just like experienced in a
bus which is accelerating. Inertial force can also be experienced at someplace else in daily life,
such as riding on a roller coaster.
An umbrella was used to illustrate the concept of centrifugal force. When an umbrella is
rotated, the water on the top surface of the umbrella will be flung outwards. It is centrifugal
force. When an object is moving in a uniform circular motion, the direction of motion
continuous change though the speed does not change. So it is an accelerated motion.
According to Newton’s Second Law, force is needed for an accelerated motion. A central force
pointing towards the centre is required for a uniform circular motion. Similar to the situation of
a bus, when the umbrella is rotated, and because of inertia, the water drops will be flung
outwards along the tangent of the path of the motion. With respect to the umbrella, the water
drops seems to move outward by a centrifugal force.
Suggested Learning Activities
Preparation before viewing the programme
1.
Teacher may discuss with students how they know about action and reaction force.
2.
Teacher may discuss with students examples in daily life which related to action and reaction
force.
Activities after viewing the programme
1.
Student may, under the guidance of teacher, review the physical concepts presented in the
programme.
2.
The teacher may discuss with students why inertial force is not a physical force.