Section 1 of 6Rules for drawing (HT only)
1
Draw the object as a simple box or dot, on its own with no surroundings.
2
Draw one arrow for every force acting on the object, starting from the object.
3
Make each arrow point in the direction the force acts.
4
Scale the arrow length to the size of the force, so a bigger force has a longer arrow.
5
Label every arrow with the name of the force, such as weight or friction.
Section 2 of 6Common forces and their directions (HT only)
| Force | Usual label | Direction |
|---|---|---|
| Weight | W | Straight down towards the Earth |
| Normal contact force (reaction) | N or R | At 90 degrees to the surface, away from it |
| Tension | T | Along the rope or string, away from the object |
| Friction | F | Along the contact surface, opposing the motion |
| Upthrust | U | Upwards, from a liquid or gas |
| Thrust / driving force | Thrust | In the direction of travel |
Section 3 of 6Worked scenarios (HT only)
Picture frame on a nail
- Picture frame on a nail Weight acts down and the two string tensions act up and outwards.
- The forces are balanced, so drawn tip to tail they form a closed triangle.
Box sliding down a slope
- Box sliding down a slope Weight acts straight down, the normal force acts perpendicular to the slope,
- and friction acts up the slope, against the motion.
Stationary boat
- Stationary boat Weight down and upthrust up are drawn the same length because the boat is not moving vertically.
Accelerating car
- Accelerating car Thrust forwards is longer than friction and drag backwards, while weight and the normal force are equal.
Section 4 of 6Reading a free body diagram (HT only)
1Identify the forces
- Identify the forces Name every arrow and check that none is missing.
2Split into directions
- Split into directions Look at the vertical and horizontal forces separately.
3Find the resultant
- Find the resultant Subtract opposite forces and add forces in the same direction in each plane.
4Describe the motion
- Describe the motion Zero resultant means constant velocity or at rest;
- a non-zero resultant means acceleration in its direction.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Free body diagram | A diagram of one object with arrows showing all the forces acting on it. |
| Normal contact force | The force from a surface that acts at 90 degrees to the surface. |
| Tension | The pulling force in a rope, string or cable. |
| Upthrust | The upward force exerted on an object by a fluid. |
| Resultant force | The overall force after all forces on an object have been combined. |
Section 6 of 6Exam technique
1
Drawing forces that the object exerts on other things; only include forces acting on the object itself.
2
Making balanced forces unequal in length - the arrows must match in size if the forces are equal.
3
Putting the normal force vertical on a slope; it is perpendicular to the surface of the slope.
4
Leaving out friction or air resistance when the question describes a moving object.