Forces, Motion & Fields
Pressure in Liquids
Hydrostatic pressure
Hydrostatic Pressure
The pressure in a liquid depends on the depth and the density of the liquid. Unlike gases, liquids are not easily compressed.
Pressure (Pa) = Density (kg/m³) × Height (m) × g (N/kg)
Key point: Pressure increases with depth - the deeper you go, the greater the pressure from the water above.
Important: Pressure acts in ALL directions in a liquid - not just downwards!
Pressure and Depth
Water pressure increases with depth. This is why dams are thicker at the bottom!
Moments
Turning effects of forces
What is a Moment?
A moment is the turning effect of a force. It depends on the size of the force AND the distance from the pivot point.
Moment (Nm) = Force (N) × Perpendicular Distance from Pivot (m)
Unit: Newton-metres (Nm)
Key: Distance must be perpendicular (at right angles) to the line of action of the force.
Example: Opening a Door
Why is the handle at the edge, not near the hinges?
Near hinges (small distance): Large force needed to create same moment
Far from hinges (large distance): Small force creates the same moment - easier to open!
Calculate: Force = 10 N, Distance = 0.8 m
Moment
= 10 × 0.8 = 8 Nm
Levers
Force multipliers
The Principle of Moments
For an object to be balanced (in equilibrium), the total clockwise moment must equal the total anticlockwise moment:
Force × Distance (left) = Force × Distance (right)
Balanced Lever
Anticlockwise: 20N × 1m = 20 Nm | Clockwise: 10N × 2m = 20 Nm | Balanced!
Magnets
Permanent magnets and magnetic materials
Magnetic Materials
Only a few elements are magnetic:
- Iron - most common magnetic material
- Nickel - used in coins and alloys
- Cobalt - used in magnets and alloys
Steel (mostly iron) is magnetic, but aluminium, copper, and gold are NOT.
Magnetic vs Non-Magnetic
Induced Magnetism
When a magnetic material is brought near a permanent magnet, it becomes a temporary magnet itself. This is called induced magnetism.
Attraction: Unlike poles induce opposite poles - the materials attract!
Temporary: Induced magnetism is lost when the permanent magnet is removed.
Electromagnets
Magnets we can switch on and off
How Electromagnets Work
When an electric current flows through a wire, it creates a magnetic field. Wrapping the wire into a coil (solenoid) makes the field stronger.
Increasing Electromagnet Strength:
- • More coils: Stronger magnetic field
- • More current: Stronger magnetic field
- • Iron core: Much stronger than air core
Scrapyard Cranes
Pick up and release scrap metal by switching the current on and off.
Electric Bells
Rapid on/off switching creates the ringing sound.
Loudspeakers
Electromagnet interacts with permanent magnet to vibrate the cone.
Maglev Trains
Float above tracks using electromagnetic levitation.
Hooke's Law
Springs and elastic deformation
Hooke's Law
The extension of a spring is directly proportional to the force applied, as long as the elastic limit is not exceeded.
Force (N) = Spring Constant (N/m) × Extension (m)
F = k × x
Spring constant (k): A measure of stiffness. Higher k = stiffer spring.
Elastic limit: If exceeded, the spring won't return to original length.
Spring Extension
No load
Original length
Under load
Extended
Example: If k = 100 N/m and F = 5 N, then x = 5 ÷ 100 = 0.05 m = 5 cm extension
Practice Questions
Test your understanding
Moments Calculation [3 marks]
A force of 15 N is applied 0.4 m from a pivot. Calculate the moment produced.
Answer:
Formula: Moment = Force × Distance [1 mark]
Calculation: Moment = 15 × 0.4 [1 mark]
Answer: Moment = 6 Nm [1 mark]
Balanced Lever [3 marks]
A see-saw has a 400 N child sitting 1.5 m from the pivot. How far from the pivot should a 600 N adult sit to balance it?
Answer:
Principle: Anticlockwise moment = Clockwise moment [1 mark]
Equation: 400 × 1.5 = 600 × distance
600 = 600 × distance [1 mark]
Answer: Distance = 1 m from pivot [1 mark]
Electromagnets [3 marks]
Explain two ways to make an electromagnet stronger.
Answer (any two for 3 marks):
• Increase the current: More current creates a stronger magnetic field (1.5 marks)
• Increase the number of coils: More coils concentrate and strengthen the magnetic field (1.5 marks)
• Use an iron core: Iron is easily magnetised and greatly increases the magnetic field strength compared to an air core (1.5 marks)
Hooke's Law [3 marks]
A spring has a spring constant of 200 N/m. Calculate the force needed to extend it by 5 cm.
Answer:
Convert units: 5 cm = 0.05 m [1 mark]
Formula: F = k × x
F = 200 × 0.05 [1 mark]
Answer: F = 10 N [1 mark]
iLowerSecondary Science Complete!
You've completed all three years of iLowerSecondary Science. Well done!