A heavy sled is being pulled by two people as shown in the figure. The coefficie
ID: 1773023 • Letter: A
Question
A heavy sled is being pulled by two people as shown in the figure. The coefficient of static friction between the sled and the ground is ?s = 0.587, and the kinetic friction coefficient is ?k = 0.451. The combined mass of the sled and its load is m = 381 kg. The ropes are separated by an angle ? = 25°, and they make an angle ? = 30.4° with the horizontal. Assuming both ropes pull equally hard, what is the minimum rope tension required to get the sled moving? If this rope tension is maintained after the sled starts moving, what is the sled's acceleration?
Explanation / Answer
Solution:
The force that causes the sled is equal to the sum of the horizontal components of the tension forces. The force that causes the sled is the friction force. There are two forces that affect the friction force. The vertical components of the tension forces are the upward forces. The total weight is the downward force. To get the sled moving, the horizontal components of the tension forces must be equal to the static force.
a)Horizontal components of the tension forces = T * cos 30.4
To determine the component of these forces that are parallel to the direction the sled is moving, multiply be cos 24.
T * cos 30.4 * cos 25
Since there are two ropes, the total force is 2 * T * cos 30.4 * cos 25
Vertical components of the tension forces = T * sin 30.4
Since there are two ropes, the total force is 2 * T * sin 30.4
Weight = 381 * 9.8 = 3733.8 N
Net downward force = 3733.8 – 2 * T * sin 30.4
Ff = 0.587 * (3733.8 – 2 * T * sin 30.4)
Ff = 2191.7406 – 1.174 * T * sin 30.4
Set the two forces equal to each other and solve for T.
2 * T * cos 30.4 * cos 25 = 2191.7406 – 1.174 * T * sin 30.4
2 * T * cos 30.4 * cos 25 + 1.174 * T * sin 30.4 = 2191.7406
T * (2 * cos 30.4 * cos 25 + 1.174 * sin 30.4) = 1846.5846
T = 2191.7406 ÷ (2 * cos 30.4 * cos 25 + 1.174 * sin 30.4) = 1015.875513 N
This is approximately 1016 N.
b)
2 * T * cos 30.4 * cos 25 = 2 * 1015875513 * cos 30.4 * cos 24
= 1588.225578
This is approximately 1,588 N
The only difference is the coefficient of friction.
Ff = 0.587 * (3733.8 – 2 * T * sin 30.4)
Ff = 0.395 * (3733.8 – 2 * 1015.875513 * sin 30.4) = 1068.737825 N
This is approximately 1069 N.
Net force = 519.4877529 N
To determine the acceleration, divide by the mass.
a = 519.4877529 ÷ 381
This is approximately 1.36 m/s2.
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