CHAPTER THREE FORCES
Force is a push or a pull. It is that which changes a body’s state of motion or shape. The SI unit for force is the Newton (N). It is a vector quantity represented by the following symbol.
Types of forces
Gravitational force – this is the force of attraction between two bodies of given masses.
Earth’s gravitational force is the force which pulls a body towards its center. This pull of gravity is called weight.
Force of friction – this is a force which opposes the relative motion of two surfaces in contact with each other. Friction in fluids is known as viscosity.
Tension force – this is the pull or compression of a string or spring at both its ends.
Upthrust force – this is the upward force acting on an object immersed in a fluid.
Cohesive and adhesive forces – cohesive is the force of attraction of molecules of the same kind while adhesive is the force of attraction of molecules of different kinds.
Magnetic force – this is a force which causes attraction or repulsion in a magnet.
Electrostatic force – this is the force of attraction or repulsion of static charges.
Centripetal force – this is a force which constrains a body to move in a circular orbit or path.
Surface tension – this is the force which causes the surface of a liquid to behave like a stretched skin. This force is cohesive.
Factors affecting surface tension
Impurities – they reduce the surface tension of a liquid, for example, the addition of detergent.
Temperature – a rise in temperature reduces surface tension by weakening intermolecular forces.
Mass and weight
Mass is the amount of matter contained in a substance, while weight is the pull of gravity on an object. The SI unit for mass is the kilogram (kg), while weight is measured in newtons (N). Mass is constant regardless of location, while weight changes with location. The relationship between mass and weight is given by the formula W = mg, where g is the gravitational acceleration.
Differences between mass and weight
| Mass | Weight |
| It is the quantity of matter in a body. | It is the pull of gravity on a body. |
| It is measured in kilograms. | It is measured in newtons. |
| It is the same everywhere. | It changes from place to place. |
| It is measured using a beam balance. | Measured using a spring balance. |
| Has magnitude only. | Has both magnitude and direction. |
Example
An astronaut weighs 900 N on Earth. On the moon, he weighs 150 N. Calculate the moon’s gravitational strength. (Take g = 10 N/kg).
Solution:
Moon’s gravitational strength = weight of astronaut on the moon / mass of astronaut.
= 150 / 90 = 1.67 Nkg-1.
Measuring force
We use a spring balance to measure force. A spring balance is an instrument that uses the extension of a spring to measure forces.
Example
The length of a spring is 16.0 cm. Its length becomes 20.0 cm when supporting a weight of 5.0 N. Calculate the length of the spring when supporting a weight of:
a) 2.5 N b) 6.0 N c) 200 N
Solution:
5 N causes an extension of 4.0 cm, therefore 1.0 cm causes an extension of 4 / 5 = 0.8 cm.
2.5 N => 2.5 × 0.8 = 2.0 cm; therefore, length becomes 16.0 + 2.0 = 18.0 cm.
6.0 N => 6.0 × 0.8 = 4.8 cm; therefore, length becomes 16.0 + 4.8 = 20.8 cm.
200 N => 200 × 0.8 = 160.0 cm; therefore, length becomes 16.0 + 160.0 = 176.0 cm.
Vector and scalar quantities
A scalar quantity is a quantity which has magnitude (size) only. Examples include distance, mass, and speed.
A vector quantity is a quantity which has both magnitude and direction. Examples include displacement, weight, and velocity.
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