07 2-1 (g) Apparent weight with changing acceleration
Idea
- A person in a lift presses on the floor with force when the lift accelerates upward with acceleration .
- If varies with time, so does ; just replace by the function.
✍️ Worked examples
Example 19:
- A lift accelerates upwards with m/s.
- A man of mass 60 kg stands in it.
- Find the pressure on the floor at s in kg·wt, and the speed of the lift at that time (starting from rest).
Solution:
- At : m/s.
- N kg·wt.
- m/s.
Common mistakes
- Using when the acceleration is not constant.
- Forgetting the constant of integration, or using the wrong initial condition.
- Mixing the forms and .
- Confusing displacement with distance when the velocity changes sign.
- Mixing units (grams with newtons, or cm with m).