Newton's Third Law — Question 6

Question 6

A trailer reaction does not cancel external traction

A motorized cart pulls a trailer along level ground at constant speed. Ground pushes the cart forward with 500 N. Ground and air together resist the trailer with 300 N and the cart with 200 N.

1. Find the tow-link force on the trailer.

2. List the horizontal forces on the cart and verify balance.

3. Why does the trailer’s backward pull not prevent forward travel?

Question 6 - Solution

A trailer reaction does not cancel external traction

(a) Balance the trailer.

$$T-300=0\ \Rightarrow\ \boxed{T=300\,\mathrm N}$$ The tow link pulls the trailer forward with 300 N to balance its 300 N resistance.

(b) Include traction and both backward forces.

$$F_{{\mathrm{net}},C}=500-200-T=500-200-300=0\,\mathrm N$$ The cart experiences +500 N traction, -200 N resistance and -300 N from the trailer, summing to zero.

(c) Use the whole interaction inventory.

$$F_{{\mathrm{ext}},C+T}=500-200-300=0\ \Rightarrow\ v=\text{constant}$$ The trailer’s pull is real, but ground traction supplies an external forward force. For the combined system the tow-link pair is internal and cancels; external forces sum to $500-200-300=0$, consistent with constant forward velocity.