1.5 Effects of forces
EFFECTS OF FORCES
A force is a push or pull that can change the motion or shape of an object.
Main Effects of Forces:
Change in speed – a force can make an object speed up or slow down.
Change in direction – a force can make an object turn or move off its path.
Change in shape – forces can deform objects, temporarily or permanently.
Equilibrium – when forces are balanced, the object stays still or moves at a constant speed.
Example Questions:
Explain how unbalanced forces affect a car moving uphill.
A box is pushed across the floor. State two effects of the applied force.
HOOKE’S LAW AND SPRING CONSTANT Hooke’s Law states that the extension of a spring is directly proportional to the force applied, as long as the limit of proportionality is not exceeded.
Formula: F = k * x Where: F = force (N) k = spring constant (N/m) x = extension (m)
Example Questions:
A spring extends 0.05 m when a 2 N force is applied. Find k.
State Hooke’s Law in words.
Limit of Proportionality: Beyond this point, force and extension are no longer proportional, and the spring may be permanently deformed.
Example Questions:
What happens when a spring is stretched beyond its limit of proportionality?
How can the limit of proportionality be shown on a graph?
FRICTION AND DRAG FORCES Friction is a resistive force between surfaces in contact. It converts kinetic energy into heat. Drag is a resistive force in fluids (air or water). It depends on speed, surface area, and fluid type. Both forces oppose motion and reduce efficiency.
Example Questions:
Why are cars streamlined?
Explain the difference between static and kinetic friction.
CENTRIPETAL FORCE Centripetal force keeps an object moving in a circular path and acts toward the center of the circle.
Formula: F = (m × v²) / r Where: m = mass (kg) v = velocity (m/s) r = radius (m)
Example Questions:
A 0.5 kg ball moves in a circle of radius 2 m at 4 m/s. Calculate the centripetal force.
What provides the centripetal force when a car turns on a flat road?
MOMENTS (TURNING EFFECT OF FORCES) A moment is the turning effect of a force about a pivot. It depends on the force applied and the perpendicular distance from the pivot.
Formula: Moment = Force × Distance (M = F × d)
Laws of Moments:
Law of Levers: F1 × d1 = F2 × d2 (in equilibrium)
Principle of Moments: total clockwise moments = total anticlockwise moments
Applications: Levers, doors, spanners, scissors, and cranes all use moments. Increasing the distance from the pivot increases the turning effect.
Example Questions:
A 400 N child sits 2 m from a seesaw pivot. Where should a 200 N child sit to balance it?
Why is a longer crowbar more effective?
Common Mistakes:
Using non-perpendicular distances.
Forgetting to convert cm to m.
Confusing balanced forces with balanced moments.
CENTRE OF GRAVITY AND TOPPLING The center of gravity (CG) is the point through which the entire weight of a body acts. For regular shapes, it is at the geometric center. For irregular shapes, it is found using the plumb line method.
Formula: Weight = mass × g
Stability and Toppling:
An object is stable if the vertical line through its CG lies within its base. It topples if the line passes outside the base.
Factors Affecting Stability:
A wide base increases stability.
A low center of gravity increases stability.
Types of Equilibrium:
Stable: returns to original position (example: cone on base).
Unstable: moves further away when disturbed (example: cone on tip).
Neutral: stays in a new position (example: ball on flat surface).
Worked Example: Q: Why are double-decker buses less stable than racing cars? A: Buses have a higher CG and a narrower base. When turning, their CG may fall outside the base, causing toppling. Racing cars have low CG and wide bases, making them more stable.
Example Questions:
How does the CG of a ladder affect its stability?
How is the CG of an irregular lamina found?
Why do racing cars have wide wheelbases?
SUMMARY OF KEY FORMULAS Density: ρ = m / V
Hooke’s Law: F = k × x
Moment: M = F × d
Law of Levers: F1 × d1 = F2 × d2
Centripetal Force: F = (m × v²) / r
Weight: W = m × g
Made by Hiba Shakeel & Zaid
curated by Yassein Abdoun