AQA GCSE Physics Foundation (8463), Foundation tier · Forces › Forces and their interactions
Practise Resultant forces. 5 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The resultant force is the single force that has the same effect as all the forces acting on an object together. You need to find the resultant of forces along a line.
Key facts
Resultant force = single force with the same effect as all the forces together
Same direction: add; opposite directions: subtract
Balanced forces: resultant = 0 N, so the object is at rest or moves at constant velocity
Notes
Forces along a line
The forces acting on an object can be replaced by a single resultant force that has the same effect as all of them together.
Forces in the same direction: add them. Forces in opposite directions: subtract, and the resultant acts in the direction of the larger force.
Example: 700 N forwards and 250 N backwards give a resultant force of 450 N forwards.
If the resultant force is zero, the forces are balanced. The object is either stationary or moving at a constant velocity.
How to answer each type of question
Calculate the resultant of forces along a line
2 marksGrade 4
Add up all the forces acting in each direction.
Subtract the smaller total from the larger.
Give the size and the direction of the resultant.
Example. A cyclist has a driving force of 180 N forwards. Air resistance of 60 N and friction of 25 N both act backwards. Calculate the resultant force on the cyclist and give its direction.
Giving a resultant force without a direction. A force is a vector.
Adding forces that act in opposite directions instead of subtracting them.
Thinking a moving object must have a resultant force on it. At constant velocity the resultant force is zero.
More tips
Memory tricks
Balanced does not mean stopped. A zero resultant means no change in velocity: still, or steady speed in a straight line.
Along a line, the resultant always acts in the direction of the bigger force.
Exam technique
Always give the size and the direction of a resultant force.
Link the resultant to the motion: zero resultant means constant velocity or at rest; a non-zero resultant means acceleration in its direction.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Define resultant forceA single force that has the same effect as all the original forces acting together.
Grade 4
Find the resultant of forces along a lineAdd forces acting in the same direction; subtract forces acting in opposite directions.
Grade 5
Recognise balanced forces (zero resultant)If the resultant force is zero, the object stays at rest or keeps moving at a steady velocity.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
The tension in the cable is increased to 3900 N. Calculate the resultant force on the crate and give its direction.
300 N upwards N
State what it means for an object when the resultant force acting on it is zero.
It stays at rest or moves at constant velocity (it is in equilibrium).
What word describes an object when the resultant force acting on it is zero?
Equilibrium.
A tractor pulls a trailer using a chain, which exerts a forward force of 2400 N on the trailer. Resistive forces (friction and air resistance) on the trailer total 2400 N. The trailer is already moving forward. Calculate the resultant force on the trailer.
0 N
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Which of these is the correct definition of the resultant force on an object? Tick (✓) one box.[1]
The largest of the forces acting on the object
The single force that has the same effect as all the forces acting on the object together
The force needed to keep the object still
The force that always acts against the object’s motion
(b) State what it means for an object when the resultant force acting on it is zero.[1]
(c) A stationary crate is pushed with a force of 90 N to the right. Friction of 90 N acts to the left. Calculate the resultant force on the crate.[2]
Show the answer and mark scheme
(a)Answer: The single force that has the same effect as all the forces acting on the object together
(b)Answer: It stays at rest or moves at constant velocity (it is in equilibrium).
the object stays at rest, or continues to move at a constant velocity (it is in equilibrium)
(c)Answer: 0 N
90 − 90
0 (N)
Question 2Medium6 marks
A crane lifts a crate using a vertical steel cable. The weight of the crate is 3600 N. Ignore air resistance.
(a) The crate hangs at rest just above the ground. What is the tension in the cable?[1]
(b) The tension in the cable is increased to 3900 N. Calculate the resultant force on the crate and give its direction.[2]
(c) Describe the motion of the crate while the tension is 3900 N.[1]
(d) Later, the crate moves upwards at a constant speed. Give the tension in the cable. Explain your answer.[2]
Show the answer and mark scheme
(a)Answer: 3600 N
3600 (N)
(b)Answer: 300 N upwards N
300 (N)
upwards
(c)Answer: It accelerates upwards.
accelerates upwards / speeds up (moving upwards)
(d)Answer: 3600 N, because the resultant force is zero at constant speed.
3600 (N)
(constant speed so) the resultant force is zero / the forces are balanced, so the tension equals the weight