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Only a net force changes motion

Years 7–10Stage 4 to Stage 5Run it

Newton's first law: an object's motion changes only while the forces on it are unbalanced. Friction is one of those forces: it acts against a push or a slide.

Demonstration: a simplified modelNot to scale1 · Held by friction

A sled with steel runners, 2.0 kg, stands on a level floor. Surface: Wood. A push of 6.00 N acts forward and friction acts 6.00 N backwards, within its limit of 9.81 N, so the net force is 0.00 N and the sled stays still. Time since the push began: 0.25 s.

Push 6 N on wood: friction matches it, up to a 9.81 N limit. Net force zero: the sled stays still.

  1. Sled
  2. Wood floor
  3. Front of the sled every 0.5 s

Key Arrows are forces along the floor, to one scale: the push forward (amber), friction backwards (red), and the net force, the two added together (dark blue). Dashed tick: the friction limit, the most friction can give on this surface.

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The idea, step by step

Not to scale

  1. A sled with steel runners, 2.0 kg, is pushed along a level floor. Surface: Wood. The push, 11.00 N forward, is past the friction limit of 9.81 N, so the sled slides, and sliding friction is 5.88 N backwards. The net force is 5.12 N forward, so the sled speeds up at 2.56 m/s². At 0.40 s it has slid 0.20 m and its speed is 1.02 m/s.
    2 · Past the limitPush 11 N, past the limit: it slides. Friction drops to 5.88 N; a net force of 5.12 N speeds it up.1 Sled · 2 Wood floor · 3 Front of the sled every 0.5 s
  2. A sled with steel runners, 2.0 kg, slides on a level floor. Surface: Wood. The push has stopped. Friction acts 5.88 N backwards, so the net force is −5.88 N, the acceleration is −2.94 m/s², and the sled slows down. At 0.75 s it has slid 0.55 m and its speed is 0.54 m/s.
    3 · Let go on woodThe push stops. Friction alone now acts, backwards, and slows the sled until it stops.1 Sled · 2 Wood floor · 3 Front of the sled every 0.5 s
  3. A sled with steel runners, 2.0 kg, slides on a level floor. Surface: Ice. The push has stopped. Friction acts 0.39 N backwards, so the net force is −0.39 N, the acceleration is −0.20 m/s², and the sled slows down. At 4.00 s it has slid 8.74 m and its speed is 1.97 m/s.
    4 · On iceThe same push on ice: friction is only 0.39 N, so the sled slows only a little.1 Sled · 2 Ice · 3 Front of the sled every 0.5 s
  4. A sled with steel runners, 2.0 kg, slides on a level floor. Surface: No friction. No force acts along the floor: there is no push now and no friction. The net force is zero, so the speed stays 2.75 m/s. At 4.00 s it has slid 10.31 m.
    5 · No frictionNo friction: after the push, no force acts along the floor, so nothing changes the speed.1 Sled · 2 Floor with no friction · 3 Front of the sled every 0.5 s
  5. A sled with steel runners, 2.0 kg, slides on a level floor. Surface: Wood. Someone pushes 5.88 N forward, just as hard as friction acts backwards, so the net force is zero and the speed stays 1.28 m/s. At 4.00 s it has slid 4.80 m.
    6 · Balanced on woodOn wood, push just as hard as friction, 5.88 N: net force zero, so the speed stays the same.1 Sled · 2 Wood floor · 3 Front of the sled every 0.5 s

The sled's motion every half second: one row for each dot on the floor, then the latest time if it falls between dots

Time since the push began (s)Distance slid (m)Speed (m/s)Change in speed since the row above (m/s)In the first figure
0.000.000.00
0.250.000.00Shown

Try it in the Lab

Practicals with real materials, each with its safety card.

With a learner

Three questions to ask

  1. What forces act on the sled along the floor while it slides after the push?
  2. Why does the sled slide further on ice than on wood?
  3. On wood, what must you do to keep the sled sliding at a steady speed, and why?

What to expect

Many learners expect the sled to slow down and stop on every surface once the push ends, even with no friction.

What to try next

Open a practical in Try it in the Lab, above, to feel friction hold a slider still, then pull it across different surfaces.

About this model

What is simplified

  • The sled's weight and the floor's push up on it balance, so they are not drawn. Only the forces along the floor are drawn, all to one scale.
  • Friction holds a still sled with whatever force it needs, up to its limit; a sliding sled feels a smaller, steady friction. The values are typical ones for steel runners on wood and on ice, and real surfaces vary.
  • No friction is an ideal. An air track or an air hockey table comes close, but a real glider still slows a little.
  • The push is steady and lasts half a second. Air resistance is left out.
  • The sled is drawn larger than to scale, so it can be seen; the distances it slides and the dots are to one scale.
  • A run goes on to the end of the time slider, so a sled at rest is seen staying at rest, unless the front of the sled reaches the end of the floor shown first.

Numbers and their sources

Review

Demonstration: a simplified model. Checked against its written sources, 26 September 2026. Not reviewed by a qualified teacher.

Curriculum references

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Reference, not a verified alignment.

All demonstrations Practicals in the Lab