Section 1 of 5Balanced and Unbalanced Forces
The resultant force decides whether an object's velocity stays the same or changes.
Balanced forces
- Forces whose resultant is zero, so the velocity of the object does not change.
Unbalanced forces
- Forces whose resultant is not zero, so the object accelerates in the direction of the resultant.
Constant velocity
- An object moving with balanced forces keeps moving at the same speed in the same direction.
No force needed
- A moving object does not need a continuing force to keep moving at constant velocity.
Acceleration rule
- An unbalanced force always produces an acceleration in the same direction as the force.
Section 2 of 5Using F = ma
This equation links resultant force, mass and acceleration for any object.
1
Find the resultant force acting on the object, and state its direction.
2
Identify the mass of the object in kilograms, converting from grams if needed.
3
Rearrange F = ma to find the unknown quantity, usually the acceleration a = F ÷ m.
4
Give the acceleration in m/s², in the same direction as the resultant force.
Section 3 of 5Effects of a Resultant Force
Effects of a Resultant Force
Speed up
- A resultant in the direction of motion increases the speed of the object.
Slow down
- A resultant opposite to the motion decreases the speed of the object.
Change direction
- A resultant at an angle to the motion changes the direction of the object's velocity.
Change shape
- Unbalanced forces can also deform an object, such as squashing or stretching it.
Section 4 of 5Worked Example
Question
- A car of mass 1200 kg has a resultant forward force of 600 N.
- Calculate its acceleration.
- Write F = ma, so a = F ÷ m.
- Substitute F = 600 N and m = 1200 kg.
- a = 600 ÷ 1200 = 0.5 m/s².
- The acceleration is in the forward direction.
Answer The acceleration of the car is 0.5 m/s² forward.
Section 5 of 5Exam Checklist & Tips
1.2.4 – Key Points for the Exam
- Newton The unit of force, equal to 1 kg m/s².
- Unbalanced force A resultant force that is not zero, which causes acceleration.
- Newton's second law The acceleration of an object is proportional to the resultant force and inversely proportional to its mass.
- Balanced force Forces whose resultant is zero, so they do not change the motion of an object.
Common Mistakes
- Believing that a moving object needs a force to keep moving at constant velocity;
- only a change needs a resultant.
- Rearranging F = ma incorrectly, for example writing a = F × m instead of F ÷ m.
- Using a mass in grams directly in F = ma, which gives an answer that is 1000 times too large.
- Omitting the unit of acceleration, m/s², or forgetting the direction of the acceleration.