1. How do Forces Affect Motion?
A force is a push or pull on an object. The combination of all the forces on an object is the net force.
The net force determines how an object will accelerate.
Sometimes forces combine together, producing a greater force.
Sometimes forces counteract each other, making a weaker force.

2. Forces in the Same Direction
When forces are applied in the same direction, the net force is found by adding the forces.
The net force on the box below is 26 N to the right.
1. Predict How would the acceleration of the box change if both dogs in the figure pushed with a force of 10 N?

3. Forces in Opposite Directions
When forces are applied in opposite directions, the net force is found by subtracting the forces.
The net force on the box below is 0 N.
2. Calculate How would the forces have to change to make the box move to the left?

4. When the net force is zero, then an object’s motion will not change.
Any nonzero net force causes a change in the object’s motion. This can result in an object speeding up, slowing down, turning, or even stopping and then reversing direction.