← Newton's three laws

Predict the motion when the net force vanishes

A deep-space probe burns its engine for weeks, gaining speed the whole time. Then the engine shuts off. Does the probe drift to a stop?

On Earth, things that lose their driving force seem to stop on their own. Far from all friction and drag, the honest answer shows itself.

the probe, tick by tickpositions at equal time ticksgrowing gaps: the probe speeds upforce from the engine on the probe
Engine on: a net force acts, and the gaps grow. The velocity keeps changing.

While the engine burns, a net force acts on the probe, so its velocity keeps changing: the probe speeds up.

At the instant the engine shuts off, the net force on the probe drops to zero. In deep space there is no friction and no drag to take its place.

the same probe, after shutoffpositions at equal time ticksequal gaps: it keeps the velocity it had at shutoff
Engine off: the net force is zero, and the equal gaps show the probe keeping the velocity it had at shutoff.

The probe keeps the velocity it has at that instant, because an object's velocity persists on its own (inertia); forces are needed only to change velocity, not to maintain it.

It does not slow down. Slowing is a change in velocity, and a change needs a net force.

It does not keep speeding up either. Speeding up is also a change, and the force that was causing it is gone.

An air-track glider is towed until it moves at 2.0 m/s, and the tow thread is then released. The track exerts no friction. What is the glider's motion afterward?

A rope tows a sled from rest over ice with negligible friction, and the sled is speeding up. The rope comes loose at the instant the sled moves at 3.5 m/s. What is the sled's motion afterward?

So the rule for a vanished net force: the object continues at whatever velocity it had at the instant the net force became zero.

MomentNet force on the probeWhat the velocity doesengine burningnonzero, forwardkeeps increasingafter shutoffzerostays at the shutoff value
The whole lesson in two rows: a nonzero net force changes the velocity; a zero net force leaves it exactly where it was.

Saskia says: a cargo capsule released from a rocket in deep space, far from any planet, keeps moving at the velocity it had at release, with nothing driving it. Is she correct?

When the net force on a moving object drops to zero, the object keeps the velocity it had at that instant.

A fan cart speeds up on a frictionless track while its fan runs. The fan is switched off at the instant the cart moves at 1.4 m/s. Describe the cart's motion afterward.

While the fan ran, a net force acted on the cart, so its velocity kept increasing.

With the fan off, the horizontal force is gone, and the frictionless track adds none.

So the net force on the cart is zero, and the cart keeps the velocity it had at that instant.

The cart continues at a constant 1.4 m/s.

Your turn

A probe coasts at 7.0 km/s with its engines off, far from any planet. Which statement is correct?

A rope tows a skater, who is speeding up, and the rope is let go at the instant she moves at 4.2 m/s. The ice exerts negligible friction. Which statement is correct?

Which general statement is correct about a moving object whose net force becomes zero?