Question

• A 60.0-kg person jumps onto the floor from a height of 3.00 m. If he lands stiffly (with his knee joints compressing by 0.500 cm), calculate the force on the knee joints.
Strategy
This person’s energy is brought to zero in this situation by the work done on him by the floor as he stops. The initial PEg is transformed into KE as he falls. The work done by the floor reduces this kinetic energy to zero.
Solution
The work done on the person by the floor as he stops is given by W = Fd cos θ = −Fd, with a minus sign because the displacement while stopping and the force from floor are in opposite directions (cos θ = cos 180º = −1). The floor removes energy from the system, so it does negative work.
The kinetic energy the person has upon reaching the floor is the amount of potential energy lost by falling through height h: KE = −ΔPEg = −mgh.
The distance d that the person’s knees bend is much smaller than the height h of the fall, so the additional change in gravitational potential energy during the knee bend is ignored.
The work W done by the floor on the person stops the person and brings the person’s kinetic energy to zero: W = −KE = mgh.
Combining this equation with the expression for W gives −Fd = mgh.
His shape and weather conditions allow him to move uphill at a speed of v1=0,5m/s. a) Calculate how many minutes it will take to reach the destination if he starts at altitude of 18 m, the Fugleberget mountain height is 568 m a.s.l. and the average inclination of the slope is a = 30°. b) How long will take the descent from the mountain if the velocity is v2=0,7m/s?

Answer 1

60+3500=3560

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Answer 2

5 ms because it is V2;and the whether