Resultant Forces
Get to grips with Resultant Forces for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A single resultant force can replace several forces on an object, and a zero resultant means the forces are balanced.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
A number of forces acting on an object can be replaced by a single force that has the same effect. This is called the resultant force.
For forces acting along the same line, add those in the same direction and subtract those in opposite directions. Two forces of 10 N right and 4 N left give a resultant of 6 N to the right.
When the resultant force on an object is zero, the forces are balanced. A stationary object stays stationary and a moving object continues at constant velocity. When the resultant is not zero the forces are unbalanced, and the object accelerates in the direction of the resultant.
Revision notes
Finding the resultant
For forces along the same line, add those acting in the same direction and subtract those acting in opposite directions.
Always state the direction as well as the size. A resultant of 6 N is incomplete; 6 N to the right is a full answer.
Balanced forces
When the resultant force is zero the forces are balanced.
A stationary object remains stationary. A moving object continues at the same speed in the same direction — constant velocity. This is Newton's first law.
Unbalanced forces
When the resultant force is not zero, the object accelerates in the direction of the resultant.
The acceleration may be a change in speed, a change in direction, or both. A larger resultant force produces a greater acceleration for the same mass.
Key points
- A resultant force replaces several forces.
- Add forces in the same direction.
- Subtract forces in opposite directions.
- Zero resultant means balanced forces.
- Balanced forces give constant velocity.
- An unbalanced force causes acceleration.
Worked examples
Example 1
A 12 N force acts to the right and a 5 N force acts to the left. Calculate the resultant force. [2 marks]
Model answer
The forces act in opposite directions, so subtract: 12 − 5 (1 mark) = 7 N to the right (1 mark).
Example 2
Explain what happens to a moving object when the resultant force on it is zero. [2 marks]
Model answer
It continues to move at the same speed in the same direction (1 mark), because there is no resultant force to change its velocity (1 mark).
Example 3
Two forces of 8 N and 3 N act on an object in the same direction. Calculate the resultant. [2 marks]
Model answer
The forces act in the same direction, so add them: 8 + 3 (1 mark) = 11 N in that direction (1 mark).
Common mistakes
Omitting the direction of the resultant.
A resultant force is a vector, so the direction must be stated.
Saying a balanced object must be stationary.
It may also be moving at constant velocity.
Adding forces in opposite directions.
Subtract when they oppose each other.
Saying zero resultant means no forces act.
Forces may act but cancel each other out.
Exam tips
- Always state the direction of a resultant force.
- Remember balanced can mean moving at constant velocity.
- Draw a diagram with arrows before calculating.
- Link unbalanced forces to acceleration.
Key terms
- Resultant force
- The single force with the same effect as several forces.
- Balanced
- Having a resultant force of zero.
- Constant velocity
- Unchanging speed and direction.
- Acceleration
- A change in velocity, caused by an unbalanced force.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.