01 1 Momentum and its change
Momentum
- Momentum of a moving body is a vector with the direction of its velocity, equal to mass times velocity.
- In rectilinear motion use algebraic measures: .
- Unit in SI: . For constant mass, is proportional to , so it is called linear momentum.
Change of momentum
- If the velocity changes from to in a given interval:
- If the acceleration is given as a function of time, .
✍️ Worked examples
Example 1: A bike of mass kg moves east at m/s. Find its momentum.
Solution:
- kg}\cdot\text{m/s towards the east.
Example 2: A car of mass kg has position metres. Find the magnitude of its momentum at the start and at s.
Solution:
- .
- At : , so ; the magnitude is kg}\cdot\text{m/s.
- At : , so kg}\cdot\text{m/s.
Example 3:
- A g rubber ball falls from cm onto a horizontal surface and rebounds to cm.
- Find the magnitude of the change in its momentum.
Solution:
- Take upward as positive and m/s.
- Before impact: m/s.
- After impact: m/s.
- kg}\cdot\text{m/s.
Example 4: The acceleration of a body of mass kg is m/s. Find its change of momentum for .
Solution:
- .
- kg}\cdot\text{m/s.