1. Balanced and Unbalanced Forces
14 July 2014 Objectives: To look at balanced or unbalanced forces and describe what will happen to the objects involved.

2. Starter (5 mins)
Show some photographs of objects, some stationary and some clearly in motion.
Ask the students to describe/draw the forces acting on the objects and explain how they know that the objects are moving

3. Moving or Stationary?

4. Force pairs Teacher Notes
Encourage pupils to predict the outcome of each simulation.

5. Balanced and unbalanced forces
Imagine a car travelling at a constant speed of 50 km/h.
The engine provides sufficient force to balance all the frictional forces that are acting to decrease the speed.
500 N
500 N

6. Balanced and unbalanced forces
A crosswind acting on the car produces a sideways force.
500 N
500 N
100 N
cross wind
The crosswind causes the direction of the car to change – this happens because the sideways forces on the car are not balanced. The car will veer sideways.
If the car turns right so that the wind is now behind the car, what will happen to the speed?

7. Resultant force
The sum effect of more than one force is called the resultant force.
The resultant force is calculated by working out the difference between opposing forces in each direction.
What is the resultant force on this truck?
A resultant force of 100 N is accelerating the truck.
400 N
500 N

8. Balanced and unbalanced forces – summary
If the forces on an object are balanced:
and the object is stationary, it will remain stationary
and the object is moving, it will continue to move at the same speed and in a straight line.
In other words, the object will continue to do what it is already doing without any change.
If the forces are unbalanced, two things can happen:
The speed can change. This is called acceleration.
The direction of motion can change.

9. Resultant forces – question 1
1. What is the resultant force on the satellite?
5 N 5 N
Resultant force = 20 N – 10 N = 10 N down
The satellite will accelerate downwards.
20 N

10. Resultant forces – question 2
2. What is the resultant force on the bird?
The forces acting in each direction horizontally are equal in size, so there is no resultant force in this direction.
Resultant force = 5 N – 5 N = 0 N
The vertical forces are not balanced, the bird will accelerate in a downwards direction.
5 N
5 N
Resultant force = 5 N – 0 N = 5 N down
5 N

11. Practical
The students can now attempt a range of ‘force challenges’ using their construction materials. Suggestions include:
Which team can hold a 50 g mass highest?
Which team can suspend a 20 g mass furthest out from the edge of a desk?
Which team can build the longest self-supporting bridge?
Which team can build a boat that can support the greatest mass?

12. Plenary
Test the students’ designs to destruction by increasing the loads on them.
The students should describe what is happening at the weaker parts of the structure as they collapse.