Newton's laws, college intro — final

Series final

Newton's laws, college intro — final

8.01SC · Newton's Laws of Motion · Age 18

Both sittings. Inertial frames and ΣF = dp/dt, then interactions, FBDs, and systems. Every concept from the lecture.

This series must show: inertial frames / first law as existence claim · ΣF = dp/dt = m a · superposition of forces · inertial mass; momentum form · Newton's third law as a vector identity F₁₂ = −F₂₁ · pairs never cancel on one FBD · free-body diagram procedure · weight vs mass, W = mg · normal force and tension as contact interactions · systems: walking, horse and cart, internal pairs drop out

Diagnostic

1. The physical content of the first law is best read as…

AHeavy things are harder to liftBThere exist inertial frames, in which free particles have constant velocityCForce is proportional to speedDEvery frame, including accelerating cars, may use ΣF = ma with only real forces

2. During a 40 N collision of a 0.15 kg baseball and a 5 kg bat, the force of ball on bat is…

AMuch less than 40 N, because the bat is heavierB40 N, opposite the force of bat on ball, on a different FBDCZero; bats don't feel baseballsD40 N on the ball as well, so they cancel and no one accelerates

3. A hanging lamp is at rest. On the lamp's FBD you draw tension up and weight down. The partner of the tension is…

AAlso on the lamp, so they cancelBThe lamp's pull on the cable (or ceiling-cable system), on that other FBDCThe weight, which is the same forceDFriction

4. A 70 kg person in an elevator with a = 2 m/s² up, g = 9.8. The scale reading is nearest…

A70 kgB686 NC826 ND546 N

5. For the system 'horse + cart', the horse–cart pair…

AIs the reason the cart cannot startBCancels in ΣF_ext; the ground's push on the hooves can exceed friction on the wheelsCBelongs on the cart's FBD twiceDViolates the third law for living things

6. Inertial mass is measured by…

AA scale in an elevatorBThe acceleration a known net force produces, independent of the gravitational fieldCVolumeDThe pair F₁₂

Show your work

7. 7. Skater A (40 kg) and B (80 kg) push; the pair is 20 N. Find each acceleration. Then write ΣF_ext on the system A+B, assuming ice is frictionless, and say why that does not contradict the pair.

8. 8. Block on a table in a truck braking at 3 m/s². The block does not slide. m = 5 kg. In the road frame: what horizontal force is on the block, whose pair is it, and may you use ΣF = ma with only real forces in the truck frame?

In your own words

In a paragraph a recitation instructor would accept: how do the three laws, an FBD, and a system budget fit together? Name one mistake that mixes two of those tools.

A situation you have not seen