In an inertial frame, zero resultant external force preserves velocity. The trolley is modelled with constant mass 2 kg, no friction and no rotation. Each test follows a = (F_right − F_left)/m, v = v₀ + at and displacement = v₀t + ½at² over three seconds. The unbalanced 8 N/4 N test accelerates from rest; the two balanced 4 N/4 N tests preserve initial velocities of 0 or +2 m/s. A common fixed ruler makes the tests comparable.
Forces, motion and energy · Discover & try
Can balanced forces act on a moving object?
Reading level 5 of 5 · AdvancedChange
Word help
Net force
The combined force after directions are taken into account. Here, equal left and right pushes cancel.
Velocity
Speed together with direction. A constant velocity means that neither changes.
Acceleration
A change in velocity over time: speeding up, slowing down or changing direction.
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See how it works
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One 2 kg trolley, two opposing pulls and the same ruler in every test. Friction is excluded.
What to look for
- Which side has the stronger pull? Follow the arrows on the same trolley.
- Compare equal pulls with the trolley still, then already rolling. Does its speed change?
- Watch one-second position marks: equal gaps mean constant speed; growing gaps show speeding up.
Explain what you noticed
A different cart moves right while experiencing 5 N left and 5 N right, with no other horizontal force. What happens next?
Keep this idea: Balanced pulls do not change motion: a still trolley stays still, and a rolling trolley keeps the same speed and direction.
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