04 2-1 (d) Acceleration as a function of velocity
Method
- If , separate in time: , or in distance: .
- A resistance proportional to the velocity gives and an exponential decay.
✍️ Worked examples
Example 10:
- A boat of mass moving at experiences a resistance (where is constant) and no other horizontal force.
- Show that and that the distance travelled before it is brought to rest is .
Solution:
- , so .
- , so .
- Using we get , so .
- when . The velocity tends to zero only as , but the distance is finite.
Example 11:
- A body of mass 3 kg moves in a line with initial velocity 2 m/s under the force N, being its velocity.
- Find the time to reach 6 m/s and the distance travelled meanwhile.
Solution:
- , so .
- Integrate: . At , : .
- At : , so s.
- For the distance use , so .
- .
- m.
Example 12:
- A car of mass 1000 kg has an engine force N and resistance N.
- Starting from rest, find in terms of and its limiting speed.
Solution:
- , so .
- Integrate: . At , gives .
- , so .
- As the speed approaches the limiting speed m/s (where driving force equals resistance).