WebLab STEM

Physics · 3. Forces, motion and energy

Resultant force, acceleration, work and energy

Connect force diagrams to motion and test energy transfers.

40 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

How do forces change motion, and where does the energy go?

Curriculum coverage

Vietnam 2018 and IGCSE motion, forces and energy.

Model scope and limitations

The model treats the object as a particle with simplified friction and no deformation.

Learning objectives

  • Draw and combine forces along one axis.
  • Apply ΣF = ma to the resultant force.
  • Distinguish work, kinetic energy and dissipated energy.

Prerequisite knowledge

  • Speed, acceleration and basic vectors.
  • Mass and distance conversions.

Core knowledge

Resultant force

Acceleration follows the resultant force, not necessarily the velocity.

Work and kinetic energy

Net work equals the change in kinetic energy: Wnet = ΔEk.

Energy conservation

Energy is not lost; friction transfers mechanical energy to internal energy.

Worked example

A 5 kg object has a 30 N pull and 10 N friction. Resultant force is 20 N, so a = 4 m/s². Over 3 m, net work is 60 J, so kinetic energy rises by 60 J.

Misconceptions to avoid

  • A moving object does not require a resultant force in the direction of motion.
  • Mass measures matter; weight is the force mg.

Virtual experiment procedure: Newton’s second law

  1. Hold mass fixed and vary pulling force across at least five values.
  2. Record resultant force and acceleration; plot a against ΣF.
  3. Repeat with another mass and compare gradients.

Safety and cautions

  • The friction model uses a Coulomb approximation and omits temperature/speed dependence.

Evidence to collect

  • A force diagram for one run and a ΣF–a table.
  • An explanation linking graph gradient to 1/m.
Open the virtual experiment

Diagnostic check

1. What is the resultant force for constant-velocity motion?

  1. 0 N
  2. Equal to weight
  3. Always positive
Show answer and explanation

Correct answer: 0 N

Acceleration is zero, so ΣF = ma = 0.

2. A 20 N pull acts right and 6 N friction left. Resultant?

  1. 14 N right
  2. 26 N right
  3. 14 N left
Show answer and explanation

Correct answer: 14 N right

Taking right as positive: 20 − 6 = 14 N.

3. What is the unit of work?

  1. J
  2. W
  3. N/s
Show answer and explanation

Correct answer: J

One joule equals one newton-metre.

Application practice

1. A 12 N resultant acts on 3 kg. Acceleration?

  1. 4 m/s²
  2. 36 m/s²
  3. 0.25 m/s²
Show answer and explanation

Correct answer: 4 m/s²

a = F/m = 12/3 = 4 m/s².

2. A 15 N force acts through 2 m in its direction. Work done?

  1. 30 J
  2. 7.5 J
  3. 17 J
Show answer and explanation

Correct answer: 30 J

W = Fs = 15 × 2 = 30 J.

3. When friction slows an object, mechanical energy mainly becomes what?

  1. Internal energy
  2. Mass
  3. Charge
Show answer and explanation

Correct answer: Internal energy

Friction heats the object and surroundings.

Academic references

  1. OpenStax Physics — peer-reviewed high-school physics
  2. BIPM — SI Brochure, 9th edition

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

Content publisher
WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer