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University Physics I · unit UPI-04

Newton's Laws

Connect the ideas, make your working visible, and test the result. Use the hints when you need a starting point; open the full solution after your attempt.

Extra challenging questions

A selection from this pathway. Open a topic below for its complete practice and challenge set.

Extra challenging · 01

Force Models & Inertial Frames: infer an input with uncertainty

A force model represents each interaction between two objects by a force vector whose size comes from a measured force law, and it applies in an inertial frame — one in which an object with no net interaction keeps a constant velocity. Use the supplied topic model Y_out = 2*u^2*f*(1-f)/g, where Y_out is HEIGHT ABOVE DECK, BOTH (m). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; g = 9.81 m s⁻². f is Moment in the flight t/T, dimensionless. Keep vc (Flatcar speed V) = 3 m s⁻¹; u (Throw speed u, relative to the flatcar) = 5 m s⁻¹ fixed. For this exercise use only 0.52625 ≤ f ≤ 0.89375 on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 0.95457349 m, with a bounded reading uncertainty ±0.047419 m. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown f by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.00003675, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

HEIGHT ABOVE DECK, BOTH against f

Predictions from the supplied Force Models & Inertial Frames model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): HEIGHT ABOVE DECK, BOTH / m

HEIGHT ABOVE DECK, BOTH against fPredictions from the supplied Force Models & Inertial Frames model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: f: Moment in the flight t/T. Vertical axis: HEIGHT ABOVE DECK, BOTH / m. Exact values are available in the data table below.0.45250.66490.87731.091.3020.51150.61080.710.80920.9084xy

Horizontal axis (x): f: Moment in the flight t/T

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1f: Moment in the flight t/T: 0.5262HEIGHT ABOVE DECK, BOTH / m: 1.271
Read the exact data as a table
HEIGHT ABOVE DECK, BOTH against f — supplied values
f: Moment in the flight t/THEIGHT ABOVE DECK, BOTH / m
0.526251.2706979485219163
0.6181251.2030911537971456
0.711.0494393476044854
0.80187499999999990.809742529943935
0.89374999999999990.4840007008154946

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

The model's endpoint predictions bracket the reading. Solve Y_out(f) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.

Reveal the worked solution
  1. Endpoint predictions are Y_out(0.52625) = 1.2707 m and Y_out(0.89375) = 0.484001 m. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives f_line = 0.52625 + (0.954573 − 1.2707)(0.89375 − 0.52625)/(0.484001 − 1.2707) = 0.673925. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives f = 0.750425; substituting back gives Y_out = 0.95457349 m.
  4. The interpolation error in the input is |f_line − f| = 0.0764997. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 0.90715449 m and 1.0019925 m. Sort the roots even if the response decreases: 0.731104 ≤ f ≤ 0.768359.
  6. The lower and upper input deviations are 0.019321 and 0.0179335. They differ because the inverse response is nonlinear; a symmetric error bar would only be a local approximation. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer f ≈ 0.750425; compatible interval [0.731104, 0.768359]. Endpoint interpolation gives 0.673925.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

Extra challenging · 02

Newton's First Law: infer an input with uncertainty

Newton's first law: in an inertial frame, a body on which the net external force is zero keeps a constant velocity, staying at rest if it was at rest, and any body moving with constant velocity has zero net force on it. Use the supplied topic model Y_out = f1-f2, where Y_out is NET FORCE ΣF (N). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. f1 is Rightward pull F₁, measured in N. Keep f2 (Leftward pull F₂) = 6 N; v0 (Starting velocity v₀) = 1.5 m s⁻¹ fixed. For this exercise use only 2.85 N ≤ f1 ≤ 9.15 N on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 0.693 N, with a bounded reading uncertainty ±0.36855 N. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown f1 by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.00063 N, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

NET FORCE ΣF against f1

Predictions from the supplied Newton's First Law model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): NET FORCE ΣF / N

NET FORCE ΣF against f1Predictions from the supplied Newton's First Law model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: f1: Rightward pull F₁ / N. Vertical axis: NET FORCE ΣF / N. Exact values are available in the data table below.-3.402-1.701-4.44e-161.7013.4022.5984.29967.7019.402xy

Horizontal axis (x): f1: Rightward pull F₁ / N

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1f1: Rightward pull F₁ / N: 2.85NET FORCE ΣF / N: -3.15
Read the exact data as a table
NET FORCE ΣF against f1 — supplied values
f1: Rightward pull F₁ / NNET FORCE ΣF / N
2.85-3.15
4.425-1.5750000000000002
60
7.5749999999999991.5749999999999993
9.1499999999999993.1499999999999986

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

The model's endpoint predictions bracket the reading. Solve Y_out(f1) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.

Reveal the worked solution
  1. Endpoint predictions are Y_out(2.85) = -3.15 N and Y_out(9.15) = 3.15 N. The target lies between them. On the selected branch the response is strictly increasing.
  2. Endpoint interpolation gives f1_line = 2.85 + (0.693 − -3.15)(9.15 − 2.85)/(3.15 − -3.15) = 6.693 N. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives f1 = 6.693 N; substituting back gives Y_out = 0.693 N.
  4. The interpolation error in the input is |f1_line − f1| = 0 N. This is within the requested numerical tolerance.
  5. Invert both limits Y_out = 0.32445 N and 1.06155 N. Sort the roots even if the response decreases: 6.32445 N ≤ f1 ≤ 7.06155 N.
  6. The lower and upper input deviations are 0.36855 N and 0.36855 N. They agree on this branch; this does not establish linearity for changes in other inputs or outside the interval. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer f1 ≈ 6.693 N; compatible interval [6.32445, 7.06155] N. Endpoint interpolation gives 6.693 N.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

Extra challenging · 03

Newton's Second Law: infer an input with uncertainty

Newton's second law says that, in an inertial frame, the vector sum of the external forces on a body equals the product of its inertial mass and its acceleration, so a = ΣF/m and the acceleration points along the net force. Use the supplied topic model Y_out = hypot(5+f2*cosd(th), f2*sind(th))/m, where Y_out is ACCELERATION |a| (m s⁻²). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; hypot(a,b) means √(a²+b²); sind/cosd/tand use degrees. m is Puck mass m, measured in kg. Keep f2 (Size of second force F₂) = 4 N; th (Angle θ between F₁ and F₂) = 60 ° fixed. For this exercise use only 1.2125 kg ≤ m ≤ 2.7875 kg on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 3.593811 m s⁻², with a bounded reading uncertainty ±0.11879 m s⁻². Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown m by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.0001575 kg, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

ACCELERATION |a| against m

Predictions from the supplied Newton's Second Law model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): ACCELERATION |a| / m s⁻²

ACCELERATION |a| against mPredictions from the supplied Newton's Second Law model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: m: Puck mass m / kg. Vertical axis: ACCELERATION |a| / m s⁻². Exact values are available in the data table below.2.6563.6394.6225.6046.5871.1491.57522.4252.85xy

Horizontal axis (x): m: Puck mass m / kg

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1m: Puck mass m / kg: 1.212ACCELERATION |a| / m s⁻²: 6.441
Read the exact data as a table
ACCELERATION |a| against m — supplied values
m: Puck mass m / kgACCELERATION |a| / m s⁻²
1.21256.441443031675593
1.60624999999999974.862412249591693
1.99999999999999983.9051248379533283
2.393753.2627674886294127
2.78749999999999962.8018832918050784

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

The model's endpoint predictions bracket the reading. Solve Y_out(m) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.

Reveal the worked solution
  1. Endpoint predictions are Y_out(1.2125) = 6.44144 m s⁻² and Y_out(2.7875) = 2.80188 m s⁻². The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives m_line = 1.2125 + (3.59381 − 6.44144)(2.7875 − 1.2125)/(2.80188 − 6.44144) = 2.4448 kg. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives m = 2.17325 kg; substituting back gives Y_out = 3.593811 m s⁻².
  4. The interpolation error in the input is |m_line − m| = 0.271548 kg. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 3.475021 m s⁻² and 3.712601 m s⁻². Sort the roots even if the response decreases: 2.10371 kg ≤ m ≤ 2.24754 kg.
  6. The lower and upper input deviations are 0.0695363 kg and 0.0742903 kg. They differ because the inverse response is nonlinear; a symmetric error bar would only be a local approximation. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer m ≈ 2.17325 kg; compatible interval [2.10371, 2.24754] kg. Endpoint interpolation gives 2.4448 kg.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

Extra challenging · 04

Newton's third law: infer an input with uncertainty

Newton's third law: when object 1 exerts a force on object 2, object 2 exerts a force on object 1 of equal magnitude and opposite direction, of the same type and at the same instant, so F(2 on 1) = −F(1 on 2). Use the supplied topic model Y_out = m*(g+a), where Y_out is HAND ON BOOK = BOOK ON HAND (N). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; g = 9.81 m s⁻². m is Book mass m, measured in kg. Keep a (Acceleration a (up is +)) = 3 m s⁻² fixed. For this exercise use only 1.2125 kg ≤ m ≤ 2.7875 kg on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 27.839332 N, with a bounded reading uncertainty ±1.1803 N. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown m by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.0001575 kg, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

HAND ON BOOK = BOOK ON HAND against m

Predictions from the supplied Newton's third law model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): HAND ON BOOK = BOOK ON HAND / N

HAND ON BOOK = BOOK ON HAND against mPredictions from the supplied Newton's third law model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: m: Book mass m / kg. Vertical axis: HAND ON BOOK = BOOK ON HAND / N. Exact values are available in the data table below.14.7320.1725.6231.0736.511.1491.57522.4252.85xy

Horizontal axis (x): m: Book mass m / kg

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1m: Book mass m / kg: 1.212HAND ON BOOK = BOOK ON HAND / N: 15.53
Read the exact data as a table
HAND ON BOOK = BOOK ON HAND against m — supplied values
m: Book mass m / kgHAND ON BOOK = BOOK ON HAND / N
1.212515.532124999999999
1.606249999999999720.5760625
1.999999999999999825.619999999999997
2.3937530.6639375
2.787499999999999635.707874999999994

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

The model's endpoint predictions bracket the reading. Solve Y_out(m) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.

Reveal the worked solution
  1. Endpoint predictions are Y_out(1.2125) = 15.5321 N and Y_out(2.7875) = 35.7079 N. The target lies between them. On the selected branch the response is strictly increasing.
  2. Endpoint interpolation gives m_line = 1.2125 + (27.8393 − 15.5321)(2.7875 − 1.2125)/(35.7079 − 15.5321) = 2.17325 kg. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives m = 2.17325 kg; substituting back gives Y_out = 27.839332 N.
  4. The interpolation error in the input is |m_line − m| = 4.44089e-16 kg. This is within the requested numerical tolerance.
  5. Invert both limits Y_out = 26.659032 N and 29.019632 N. Sort the roots even if the response decreases: 2.08111 kg ≤ m ≤ 2.26539 kg.
  6. The lower and upper input deviations are 0.092139 kg and 0.092139 kg. They agree on this branch; this does not establish linearity for changes in other inputs or outside the interval. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer m ≈ 2.17325 kg; compatible interval [2.08111, 2.26539] kg. Endpoint interpolation gives 2.17325 kg.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

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