Solutions of chapter 2
Force in and on the bodyAnswer Q5
Q.6 The action of chewing involves a third –class lever system . figure A shows the jaw and chewing muscle ;figure B is the lever diagram. M is the force supplied by the chewing muscles that close the jaw about the fulcrum F . W is the force exerted by the front teeth .Q6
Figure 1
Answer Q6( a). The sum of the torques about F = 0 . Ml1 –W(l1 +l2) =0Where l2= 3l1 Ml1 = W(l1 +3l1) M = 4 W M = 400 N 6( b). 100 N = 2 x 10 N / mІ -4 0.5 x 10 mІ
Q. 7 One first class lever system involves the extensor muscle , which exerts a force M to hold the head erect (figure2) : the force W of the weight of the head , located at its center of gravity , lies forward of the force F exerted by the first cervical vertebra . the head has a mass of about 4 kg or is about 40N.
figure2
Q7Answer Q7a
The distance between F and W =3 cm The distance between F and M =5 cm 3W = 5M M = 0.6 W W= 4 kg Or W= 40 N M= o.6 x 40 = 24 N F = W + M F= 64 NAnswer Q7 b
Answer Q.7 c
The mass of body is 70 kg The mass of head is 4 kgIn standing position = 70 – 4 = 66 kg The stress = 66 x 9.8 -4 5x 10 6 The stress = 1.3 x 10 N/mІWhich is less 1 % of the maximum compression strength .Q.8 A 50kg person jumping from a height of 1 m is travelling at 4.5 m/sec just prior to landing . Suppose she lands on a pad and stops in 0.2 sec . What maximum force will she experience ?
Answer Q 8
F = Δ (mv ) ΔtF = 50x 4.5 0.2 F =1125 NQ. 9 Estimate the force on the forehead in figure 2 if the mass of the head is 4 kg , its velocity is 15 m/sec , and the padded dash stops it in 0.002 sec .