Adding forces

The idea

Unbalanced forces give a non-zero net force: for opposite forces it equals their difference, along the larger force; for same-direction forces it equals their sum, in that direction; an object's motion depends only on the net force.

Pushing a stalled car

One person pushes a stalled car. Then a second person pushes in the same direction.

What do you think happens to the total force on the car?

• It becomes the sum of the two forces
• It stays the same as one push
• It becomes zero

Find the net force

Set the forces from the book's example: 10 N and 6 N.

One force that replaces all

Several forces on an object work together as a single net force. In the same direction, they add. In opposite directions, you subtract the smaller from the larger, and the net force points the way of the larger force. Only this net force decides how the object moves.

Unbalanced forces give a non-zero net force: for opposite forces it equals their difference, along the larger force; for same-direction forces it equals their sum, in that direction. An object's motion depends only on the net force.

Notes

Net force: the sum for forces in the same direction, the difference (along the larger) for opposite forces. Motion depends only on the net force.

Check yourself

Two forces of 10 N and 6 N act on a block in the same direction. What is the net force?

Answer: 16 N

10 N + 6 N = 16 N, in the same direction as the two forces.

Forces of 10 N and 6 N act on a block in opposite directions. What is the magnitude of the net force?

Answer: 4 N

10 N − 6 N = 4 N, along the larger force.

Opposite forces of 10 N and 6 N act on a block. In which direction is the net force?

Several forces act on an object. Its motion depends…

• Along the 6 N force. The net force points along the force of larger magnitude.
• Along the 10 N force — correct. Yes!
• There is no net force. The forces are unequal, so there is a net force of 4 N.
• on the largest single force alone. The motion depends on the net force that all the forces together give.
• on the smallest force alone. The motion depends on the net force, not on one of the forces by itself.
• only on the net force — correct. Yes!