The Real Inward Force
The real force on the object
- The centripetal-force equation gives the inward resultant that is needed.
- A question may then ask for the real force that provides this resultant.
- Draw only the real forces acting on the object. Do not add a new arrow called centripetal force.
- acts along a tight string. In the stopper diagram, tension points towards the centre.
| Situation | Real inward force |
|---|---|
| car turning on a flat road | from the road on the tyres |
| planet or satellite in a circular orbit | |
| rubber stopper on a string | tension in the string |
| rider against a spinning wall | from the wall |
- Mark the centre of the circular path.
- Draw and name every real force on the object.
- Find the vector sum towards the centre.
- Set that inward resultant equal to or .
Your turnQuick check
Name the real inward force for a car on a flat road, a satellite in orbit and a stopper on a string.
Show worked answer
A car uses friction from the road. A satellite uses gravity. A stopper uses tension.
When the inward force is too small
- If the string breaks, tension becomes zero. The stopper first moves along the tangent to the old circle.
- If a car reaches oil on a road, the largest friction force from the road may be smaller than the inward force needed for the turn.
- The car then cannot keep following the same circular path. Its path becomes less curved.
Worked example
Explaining a car leaving a bend
A car reaches oil while turning on a flat road. Explain why it can no longer follow the same circular path.
- Friction from the road provides the inward resultant force.
- The oil reduces the friction that the road can provide.
- The force is too small to produce the required centripetal acceleration, so the car does not follow the same circle.
Common mistake
A resultant is the vector sum of the real forces. It is not an extra force to add to the force diagram.