Read the net-force direction off the motion

You cannot see forces. You can see motion.

A ball rolls uphill, slowing as it climbs. The motion points uphill, yet the net force on the ball points downhill. Reading that off the motion is this lesson's skill.

Acceleration points with the net force. Acceleration also shows itself in how the velocity is changing. Chain the two, and motion becomes a net-force detector.

The net force points the way the velocity is changing, never simply the way the object is moving.

A cart speeds up as it moves right. Its velocity's change points right, so the net force on it points right.

Speeding up: the gaps grow, the change points with the motion, and so does the net force.

The same cart slows while it moves right. The change points left, against the motion, so the net force on it points left.

Slowing: the gaps shrink, the change points against the motion, and so does the net force.

A pickup truck moving north is braking smoothly. Which direction is the net force on it?

A marble rolls down a ramp, speeding up. Which direction is the net force on it?

The same cart holds a steady velocity. Nothing is changing, so the net force on it is zero.

Steady velocity: equal gaps, no change, and a net force of zero.

A freight train holds a steady 32 m/s on a straight, level track. What is the net force on it?

Now the ball on the hill. It slows as it rolls uphill, so its velocity change points downhill. The net force on it points downhill.

The same ball rolls back down, speeding up. The change still points downhill, and so does the net force.

Slowing on the way up: the velocity points up the slope, the change points down the slope, and the net force points down the slope.

So ask three questions in order: which way is it moving, is it speeding up or slowing down, and so which way is the velocity changing? The net force points that way. If nothing is changing, the net force is zero.

The net force points the way the velocity is changing, never simply the way the object is moving. An unchanging velocity means zero net force.

A westbound subway train slows as it enters a station. Which direction is the net force on it?

The train moves west and is slowing.

A slowing object's velocity change points against its motion, so the change points east.

The net force points the way the velocity is changing.

The net force on the train points east.

Your turn

An elevator moving upward slows to a stop at a floor. During the slowing, which direction is the net force on it?

A sprinter accelerates out of the blocks, moving east and gaining speed. Which direction is the net force on her?

A ball is tossed straight up and is still rising. Which direction is the net force on it?