Net Force Changes an Object's Speed or Direction
Atlas stands in a busy school hallway, pushing a heavy cart loaded with science equipment toward the lab; he leans forward with both hands on the handle while a student pushes from the opposite side, making the cart barely move as Atlas grins and explains what is happening to the forces.
- Explain what net force means and how to find it when forces act in the same or opposite directions.
- Identify whether the forces on an object are balanced or unbalanced and predict the effect on its motion.
- Predict whether an object will speed up, slow down, stay the same speed, or change direction based on the net force acting on it.
- Calculate simple net force by adding or subtracting forces given in Newtons.
- Compare how a larger net force and a smaller net force affect the same object differently.
Key terms
- Net force
- The single combined force found by adding all forces acting on an object.
- Newton
- The unit of force, abbreviated N, used to measure pushes and pulls.
- Balanced forces
- Forces that cancel to a net force of zero, causing no change in motion.
- Acceleration
- A change in motion: speeding up, slowing down, or changing direction.
Combining Forces
To find the net force you combine every push and pull acting on the object at once. Forces pointing the same way add together, while forces pointing in opposite directions subtract. For example, a 40 N forward push opposed by a 25 N backward push leaves a net force of 40 minus 25, which is 15 N forward. Drawing labeled arrows for each force makes it easy to see which add and which subtract before you do the arithmetic.
From Net Force to Acceleration
Once you know the net force you can predict the motion. A net force of zero means balanced forces and no change: the object stays still or keeps gliding at a steady speed. Any non-zero net force is unbalanced and always causes acceleration, meaning the object speeds up, slows down, or turns. The object accelerates in the direction of the net force, and a larger net force on the same object produces a larger acceleration.
Worked examples
A ball is kicked with 18 N right while air resistance pushes 3 N left; find the net force.
- The two forces point in opposite directions, so subtract them.
- Compute 18 N minus 3 N, which equals 15 N.
- The larger force points right, so the net force points right.
Answer: 15 N to the right (the ball accelerates right).
Two tug-of-war teams each pull a rope with 200 N in opposite directions; find the net force.
- The forces are equal in size and opposite in direction.
- Subtract: 200 N minus 200 N equals 0 N.
- A net force of zero means balanced forces.
Answer: 0 N — the rope does not accelerate.
Activity
Drag force arrows onto the cart and find the net force for each scenario.
Practice
Find the net force when 30 N forward and 12 N backward act on a cart.
Predict the motion of a box with only a 10 N backward friction force acting.
Common mistakes to avoid
- More forces always mean more motion.Equal opposite forces cancel to zero net force, so the object's motion does not change at all.
- A moving object must have a forward force.Forward motion can be leftover from an earlier push, so steady motion needs no current forward force.
Check your understanding
A soccer ball is kicked with 18 N of force to the right. Air resistance pushes back on it with 3 N to the left. What is the net force on the ball, and which way does it accelerate?
Two teams in a tug-of-war each pull a rope with exactly 200 N in opposite directions. What happens to the rope?
A box slides across the floor. Friction acts on it with 10 N backward, and nobody is pushing it forward. What is the net force, and what happens to the box's speed?
Recap
Net force is the combined result of all forces, found by adding same-direction forces and subtracting opposite ones; a zero net force keeps motion steady, while any unbalanced net force causes acceleration in its own direction.
Reflect
When you see something speed up, slow down, or turn, how could you figure out the direction of the net force on it?