Newton's laws, high school — final

Series final

Newton's laws, high school — final

8.01SC · Newton's Laws of Motion · Age 16

Both sittings. Inertial frames and ΣF = m a, then pairs, FBDs, mass versus weight. One object per diagram.

This series must show: inertial reference frames · Newton's first law / inertia · Newton's second law ΣF = ma · superposition / net force · inertial mass · Newton's third law F₁₂ = −F₂₁ · pairs act on different objects (do not cancel on one FBD) · free-body diagrams · mass vs weight W = mg · contact forces: normal and tension · horse-and-cart / walking as pairs

Diagnostic

1. A coffee slides 'back' on the dash as a car starts. In the sidewalk frame…

AA real force shoved the coffee backwardBThe coffee kept constant velocity; the car accelerated forward around itCGravity briefly pointed backwardDThe sidewalk is non-inertial so Newton's laws fail

2. A crate is pulled right 12 N and left 12 N. The net force is…

A24 N, because both pulls countB0, so if it was still it stays stillC12 N right, the 'stronger looking' oneDUndefined until we know the mass

3. Same net force on 1 kg and 4 kg. The 4 kg object's acceleration is…

A4 times largerB4 times smallerCThe same, because force determines speedDZero, because heavy objects cannot accelerate

4. You push a wall with 50 N. The wall pushes you…

AWith less, because walls don't tryBWith 50 N, opposite; that arrow belongs on YOU, not on the wall's FBDCWith 50 N, and the two arrows cancel so nothing happensDNot at all, unless the wall falls over

5. On a book's FBD as it sits on a table, you should draw…

AWeight down, normal up, and the book's push on the table downBWeight down and normal up only (plus any other forces ON the book)COnly weight, because sitting things have no normal forceDThe table's weight

6. A 2 kg bag on the Moon (g ≈ 1.6 m/s²). Its mass and weight are…

AMass 2 kg, weight 2 NBMass 2 kg, weight about 3.2 N downCMass 3.2 kg, weight 2 NDMass 0, because the Moon has no gravity in movies

Show your work

7. 7. m = 3 kg. Forces on it: 10 N east, 4 N west, 6 N north. Find ΣF as a vector and a as a vector. Name the frame you just treated as inertial.

   Hint: Add as vectors, then divide by m.

8. 8. Horse and cart. Why does putting both pair arrows on the cart's picture already lose, and what external force starts the system?

   Hint: Two FBDs. Then the ground.

In your own words

A classmate says 'force makes things move, and the pair cancels, and heavier means more force because of weight.' Rewrite that so it matches the lecture.

A situation you have not seen