Forces are measured in newtons (N) and have a size and a direction. When several forces act on an object, we combine them into one resultant force. Forces in the same direction add; forces in opposite directions subtract. If the resultant force is zero, the forces are balanced, and the object either stays still or keeps moving at a steady speed in a straight line.
If the forces are unbalanced, the object speeds up, slows down or changes direction, in the direction of the resultant force. Contact forces need touching, like friction, air resistance and the push of a surface. Non-contact forces act at a distance, like gravity, magnetism and electrostatic force.
Worked example
A box has 50 N pushing right and 20 N of friction to the left. What is the resultant force?
- Opposite directions, so subtract.
- 50 − 20 = 30.
Answer:The resultant is 30 N to the right, so the box speeds up.
Key words
- resultant force
- the single force that has the same effect as all the forces
- balanced
- resultant force is zero
- contact force
- needs objects to touch
- non-contact force
- acts at a distance
New questions every time.
NGSS-referenced: MS-PS2-2