## Thursday, 19 April 2012

### "Force" (In Lower Secondary Years)

"Force"

1.      A force is a push or a pull:

i.      which can cause an object to change its shape (e.g. plasticine)
(In upper secondary, you will learn about the relationship between the force applied on a spring and its extension or compression as summarized by Hooke’s Law; and elastic potential energy – please see: Form 4 Physics Chapter 2)

ii.      which can cause an object to change its state of motion in terms of:
1.      position (pushing or pulling a trolley)
2.      speed (blowing a moving ping-pong ball in the direction of its motion)
3.      direction (blowing a moving ball at an angle to its direction of motion)
(In upper secondary, you will learn about the effect of force on the state of motion of a body in greater details)

iii.      which can produce turning effect known as moment of a force.
(Moment (N m) = Force (N) x Perpendicular distance (m) from pivot to the force; and levers operates on principle of moments – It iss 1 of the simple machine – 3 types of levers: 1st class, 2nd class and 3rd class levers)

2.   Forces which exist in nature include:
i.      Gravitational force - a mass in a gravitational field experiences gravitational force. Now, in Form 4, you will learn about gravity in greater details.

ii.      Magnetic force - a magnet in a magnetic field experiences magnetic force: Like poles repel and unlike poles attract. You will learn more of this in Form 5 Chapter 3 on “Electromagnetism”.

iii.      Electrostatic force - an electric charge in an electric field experiences this force. This will be covered in Form 5 Chapter 2.

iv.      Frictional force – It exists between a mass and the surface in contact. This was covered in Form 2 Chapter 7:

1.      The direction of frictional force is always against the direction of motion of the mass.

2.      The magnitude of frictional force depends on:
a.       The mass of the body – the heavier the mass, the higher the friction; and
b.      The roughness of the surface in contact – the rougher the contact surface, the higher the friction:

i.      To increase friction, roughen the contact surface – use of rubber pad, deep treads on shoes and tyres.

ii.      To reduce friction, smoothen the contact surface – use of oil and lubricants; ball bearings, rollers and wheels; move on air or magnetic field (maglev); streamlined body and so on.

3.      One way to measure force is by the use of spring balance.

4.      The SI unit of force is newton (N)

5.      Work is done when an object is moved by a force through a distance in the direction of the force:
Work done (J) = Force (N) x Distance moved (m)

6.      Power is the rate of doing work: Power (W) = Work (J) / Time (s)

7.      The turning effect of a force about a pivot is known as the moment of a force which is the product of the force F and the perpendicular distance l of the force from the pivot
Moment = Force F x Perpendicular Distance l

8.      Moments about a pivot are balanced when:
Sum of anti-clockwise moments = Sum of clockwise moments