Calculate the resultant of forces acting in a straight line.
For higher-tier students: Draw a Freebody diagram to show forces acting on an object.
Note: This lesson focuses on calculating resultant forces, not on their effects on motion.
Key Concepts
Force: A push or pull on an object due to interaction with another object. Force is a vector quantity, meaning it has both magnitude (size) and direction.
Resultant Forces
The resultant force is a single force that represents the combined effect of all the original forces acting on an object.
To calculate the resultant force:
Subtract the smaller force from the larger force.
Example: If a man pushes a box with 20 Newtons to the right and there’s a 10 Newton friction force to the left, the resultant force is 10 Newtons to the right.
Examples of Calculating Resultant Forces
Car on Road:
Driving force: 10,000 Newtons (left)
Friction with road: 4,000 Newtons (right)
Air resistance: 5,000 Newtons (right)
Total force to the right = 9,000 Newtons
Resultant force = 10,000 N (left) - 9,000 N (right) = 1,000 N (left)
Skydiver:
Downward force (weight due to gravity): 800 Newtons
Upward force (air resistance): 800 Newtons
Forces are balanced, so the resultant force is zero.
Freebody Diagrams
A Freebody diagram simplifies representation by showing the object as a point.
Forces are depicted as arrows starting at the point.
The arrow's length indicates the force's magnitude.
The arrow's direction shows the force's direction.
Example: Airplane at Constant Velocity and Altitude
Weight acts downwards.
Lift force (equal and opposite to weight) acts upwards.
Forward thrust from engines.
Air resistance (drag) acts in the opposite direction to thrust.
At constant velocity, forward and backward forces are balanced, evident by equal arrow lengths.
Additional Resources
Revision questions on resultant forces are available in the workbook linked above.