A force diagram shows every force acting on one object, as labelled arrows that start on the object and point the way each force acts. You are asked to draw one for a stated situation, such as a box being dragged, a parachutist falling or a car slowing down.
This skill sits at the start of forces and momentum. Every later calculation, from resultant force to acceleration, depends on a correct diagram.
What is the quick method for any situation?
Follow the same four steps every time, and the diagram almost draws itself.
- Choose the object and draw it as a simple box or dot.
- Draw weight first. It always acts downwards, towards the centre of the Earth.
- Look for contact. Ask what is touching the object: a floor, a rope, a hand, the air, a wall. Each contact can give a force, such as a normal (support) force, tension, friction or air resistance.
- Label and size the arrows. Write the name of each force. Make arrows longer for larger forces and equal in length for equal forces.
Forces act on the object. If a question asks for the forces on the box, do not draw the force the box pushes down on the floor with. That force acts on the floor.
Worked example
Invented situation: a box of mass 5.0 kg is pulled along a rough floor at a steady speed by a horizontal rope. The rope pulls with 20 N. Take g = 10 N/kg, as a question would state.
Step 1, object: the box, drawn as a rectangle.
Step 2, weight: weight = mass × g = 5.0 × 10 = 50 N, downwards.
Step 3, contact with the floor: the floor pushes up on the box. This is the normal (support) force. The box is not moving vertically, so it equals the weight: 50 N upwards. The rough floor also gives friction, acting against the motion, so it points backwards.
Step 3b, contact with the rope: tension of 20 N acts forwards along the rope.
Step 4, label and size: the weight and normal force arrows are equal and long. The pull and friction arrows are equal and shorter, because the box moves at steady speed (20 N forwards, so 20 N of friction backwards).
Check: four arrows in total: weight (down), normal force (up), pull (forward), friction (backward). No arrow labelled “motion”.
The mistake to watch for
A common slip is to add a forward arrow labelled “force of motion” or “momentum” on a moving object.
Mistaken diagram: a car driving at steady speed with five arrows, one of them a large forward arrow labelled “force of motion” in addition to the driving force.
The student felt that movement needs its own force. No such force exists: the car’s forward drive already comes from the engine’s driving force.
The correction is to ask, for every arrow, “what is touching or pulling the object to cause this?” Weight comes from gravity, normal force from the surface, tension from the rope, friction and air resistance from contact with surfaces or air. If you cannot name the source, the arrow does not belong.
Check yourself
Try these on paper first, then open each answer.
1. A parachutist falls at a steady speed. Name the forces on the parachutist and compare their sizes.
Show answer
Two forces: weight downwards and air resistance upwards. The speed is steady, so the two arrows are the same length.
2. A ball has been thrown upwards and is now rising. Ignore air resistance. How many forces act on it, and which way do they point?
Show answer
One force: its weight, pointing downwards. There is no arrow for the throw, because the hand is no longer touching the ball.
3. A car of mass 1200 kg accelerates forwards along a level road. List the forces and say which horizontal arrow is longer.
Show answer
Weight (down), normal force (up), driving force (forward) and air and other resistance (backward). The driving force is the longer horizontal arrow, because the car speeds up.
Where this leads next
With a reliable diagram you can add the arrows up: move on to calculating a resultant force. The triangle and bearings reasoning board helps when two perpendicular forces need combining, and the forces and momentum practice set tests the whole module.
Some students draw the diagram correctly in class but lose arrows under exam conditions. That is the kind of habit our teachers watch for in online one-to-one Physics tuition.