P115. Past Mistakes on Net Force Solutions

Perhaps you can avoid these mistakes that students have made in the past. The most important thing you can do to avoid mistakes is to study the Guide to Solving 1D Net Force Problems and follow those guidelines.

• The directions of velocity and/or acceleration aren't shown on the picture.

• The positive direction is selected opposite that of acceleration.

• The direction of acceleration is incorrect. (Note that if the object is speeding up (slowing down), the acceleration is in the same (opposite) direction as the velocity.)

• Force diagrams and/or net force equations are missing or incomplete.

• The force diagram includes an F_net or F_app or W_app force.

• The net force equation includes an F_appor W_app force.

• Fictitious physics is used. Here are examples of physics equations that students have invented but are incorrect: F_net = ma₁ + ma₂ and F_net = -ma. (Note that you will never have multiple ma terms in an F_net equation nor will you have a negative sign in front of the ma term.)

• Force magnitudes are treated as negative. (Note that the symbols T, W, N, and f represent magnitudes, and magnitudes are positive by definition.)

• g is treated as a negative number.

• Vectors and magnitudes are mixed in a net force equation. Example: . (Note that you can add vectors to vectors and magnitudes to magnitudes, but you can't add vectors to magnitudes.)

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