PPT: Forces and Energy | Year 9 Science IGCSE (Cambridge) - Class 9 PDF Download

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Forces and Energy
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Forces and Energy
Forces and Motion: Balanced or Unbalanced?
Balanced Forces
When forces are equal in 
size and opposite in 
direction, they are 
balanced. Objects remain 
stationary or move at 
constant speed.
Unbalanced Forces
When forces are not 
equal in size or direction, 
they are unbalanced. 
Objects change their 
motion - speeding up, 
slowing down, or 
changing direction.
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Forces and Energy
Forces and Motion: Balanced or Unbalanced?
Balanced Forces
When forces are equal in 
size and opposite in 
direction, they are 
balanced. Objects remain 
stationary or move at 
constant speed.
Unbalanced Forces
When forces are not 
equal in size or direction, 
they are unbalanced. 
Objects change their 
motion - speeding up, 
slowing down, or 
changing direction.
Forces and Motion: Balanced or Unbalanced?
Force diagram: Arrows show size and direction of each force. Longer arrows represent bigger forces. For balanced 
forces, arrows are the same length and point in opposite directions. 
Vertical force balance
Consider a stationary rock. Gravity pulls it downward 
(its weight), while the ground pushes upward (contact 
force). These forces are balanced. 
Horizontal force balance
If wind blows against the rock, this pushing force is 
balanced by friction between the rock and ground. 
When all forces are balanced, the object doesn't 
move.
Page 4


Forces and Energy
Forces and Motion: Balanced or Unbalanced?
Balanced Forces
When forces are equal in 
size and opposite in 
direction, they are 
balanced. Objects remain 
stationary or move at 
constant speed.
Unbalanced Forces
When forces are not 
equal in size or direction, 
they are unbalanced. 
Objects change their 
motion - speeding up, 
slowing down, or 
changing direction.
Forces and Motion: Balanced or Unbalanced?
Force diagram: Arrows show size and direction of each force. Longer arrows represent bigger forces. For balanced 
forces, arrows are the same length and point in opposite directions. 
Vertical force balance
Consider a stationary rock. Gravity pulls it downward 
(its weight), while the ground pushes upward (contact 
force). These forces are balanced. 
Horizontal force balance
If wind blows against the rock, this pushing force is 
balanced by friction between the rock and ground. 
When all forces are balanced, the object doesn't 
move.
Starting to Move
Unbalanced Forces Cause Movement
When forces become unbalanced, objects begin to 
move in the direction of the larger force. This 
fundamental principle explains how stationary 
objects start moving.
For example, if a large vehicle pushes a rock with a 
force greater than friction, the rock will move 
sideways.
Page 5


Forces and Energy
Forces and Motion: Balanced or Unbalanced?
Balanced Forces
When forces are equal in 
size and opposite in 
direction, they are 
balanced. Objects remain 
stationary or move at 
constant speed.
Unbalanced Forces
When forces are not 
equal in size or direction, 
they are unbalanced. 
Objects change their 
motion - speeding up, 
slowing down, or 
changing direction.
Forces and Motion: Balanced or Unbalanced?
Force diagram: Arrows show size and direction of each force. Longer arrows represent bigger forces. For balanced 
forces, arrows are the same length and point in opposite directions. 
Vertical force balance
Consider a stationary rock. Gravity pulls it downward 
(its weight), while the ground pushes upward (contact 
force). These forces are balanced. 
Horizontal force balance
If wind blows against the rock, this pushing force is 
balanced by friction between the rock and ground. 
When all forces are balanced, the object doesn't 
move.
Starting to Move
Unbalanced Forces Cause Movement
When forces become unbalanced, objects begin to 
move in the direction of the larger force. This 
fundamental principle explains how stationary 
objects start moving.
For example, if a large vehicle pushes a rock with a 
force greater than friction, the rock will move 
sideways.
Slowing Down
Unbalanced forces don't just cause objects to speed up4they can also make moving objects 
slow down. A parachute demonstrates this principle perfectly. When an object is falling quickly, 
the parachute creates air resistance that exceeds the object's weight. This unbalanced force 
situation causes the object to decelerate.