1. A man stands on a scale in an elevator that has an upward acceleration a. The scale reads 960 N. When he picks up a 20kg bos, the scale reads 1200 N. Find the mass of the man, his weight, and the acceleration a.
2. A horizontal force of 100 N pushes a 12kg block up a frictionless incline that makes an angle of 25degree with the horizontal. (a) What i s the normal force that the incline exerts on the block ? (b) what is the acceleration of the block ?
3. A person in an elevator is holding a 10kg block by a cord rated to withstand a tension of 150 N. When the elvator starts up, the cord breaks. What was the minimum acceleration of the elevator ?
4. A 60kg painter stands on a 15kg platform. The platform is attached to a rope that passes through an overhead pulley. (a) To accelerate herself and the platform at 0.8 m/s^2, with what force must she pull on the rope ? (b) what force is she exerting on the rope when her speed reaches 1 m/s and she pulls the rope such that the platform has a constant speed ??
The Attempt at a Solution
1. I did Newton's 2nd law for the man and the man and the box then subtract the system of the man and box by the man to get a. However, I got it wrong.
2. I think the normal force is equal and opposite to the weight so is it -12 * 9.81 ?? For part b, should a = (Fcos25- mgsin25) / m ???
3. I use F=ma. and substitute on the left side as T-mg=ma but I get the wrong answer.
4. I combined 2 mass as one and treat it as one object. Then I use F=ma, left side is F- 75g= ma. I got the wrong answer.
I really need to do these corrections because quarter is ending in a week and I have so much to do this weekend. Thanks a lot.