Stability

An object will topple (fall over) if the line of action of its weight (the force) lies outside the base. (the line from the centre of mass)

The weight of the object causes a turning effect which makes the object topple.

If the line of action...

• lies inside the base, the car will not topple over
• lies above one edge of the base, the car is on the point of toppling
• lies outside its base, the car will topple over

The stability of the car can be increased by...

• having a lower centre of mass
• having a wider wheel base

Example of an object which is manufactured especially to prevent toppling over is tractors - they have wide wheels and their engines are kept low.

Buses and coaches have to go through tilt tests before they are allowed on the road - safety precaution.