Starting Motion

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James Scott
Mr. Freedman
Physics
10 May 2017
Starting Motion
Problems that demonstrate the advantages of a lever are popular among high school
students. One of them is the following: Model a situation in which an average person can
prevent a car from starting motion.
In most of the passenger cars, the rear wheels are the driving force. The model based
on this fact is the following: To take advantage of a lever, one has to mount a long pole on the
automobile as shown on Figure 1. The values of the model parameters are the following: The
length of the pole equals 100 m, the distance between the athlete and the vehicle is equal to
100 m, the distance between front wheels of the car and the rear ones equals 2 m, and the
mass of the automobile equals 1000 kg. Under such circumstances, the person takes the rear
wheels of the car off the ground through pulling the rod with the force which magnitude
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Figure 1
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T is such that sum of the torque at the point A equals zero. Hence, one can write the following
(see Figure 1):
T ∙ DAsin ɑ = Mg ∙ AB. (1)
Since DA = CA =100 m, ɑ = 45°. Therefore, from the equation (1), it follows that
T ≈14.14 ∙ g ≈ 138.6 N because AB=1 m and Mg=9800 N. Actually, performing
daily activities people often exert the force which magnitude is equal to the one of