PHYSICS · Stage 8 · Chapter 6: Forces, Motion and Pressure
Physics 6.6 Balanced and unbalanced forces — Resultant Force and Motion Changes
5 min read 5 parts Lesson 6 of 19
Lesson summary
Physics 6.6 Balanced and unbalanced forces — Resultant Force and Motion Changes — Alhussan International Academy — Physics (Stages 7–9) notes: Explain balanced and unbalanced forces and their effects on the motion of an object.Balanced forces — Forces of the same size in opposite…
- Resultant (net) force
- Same direction
- Opposite directions
- Zero resultant
- Stationary or steady speed
- Speeding up, slowing down or changing direction
Section 1 of 5Balanced forces
1Balanced forces4 points
- Forces of the same size in opposite directions cancel out, like a tug-of-war between equal teams.
- The object behaves as if there is no force on it.
- A floating diver's weight is balanced by the upthrust of the water.
- A standing person's weight is balanced by the reaction force of the ground.
2Effect3 points
- If the forces on an object are balanced, its motion does not change.
- A stationary object stays still.
- A moving object keeps moving at a steady speed in a straight line.
Section 2 of 5Unbalanced forces
- Unbalanced forces
- Forces that are not equal and opposite do not cancel out.
- Effect
- The motion of the object changes.
- A stationary object starts moving.
- A moving object speeds up, slows down or changes direction.
| Resultant force | Effect on motion |
|---|---|
| In the direction of motion | Speeds up |
| Opposite to the direction of motion | Slows down |
| At an angle or sideways | Changes direction |
| Zero | No change: at rest or constant speed |
Section 3 of 5Resultant force
- Resultant (net) force = the overall effect of all the forces on an object.
- Same direction
- Add the forces.
- Opposite directions
- Subtract the smaller force from the larger one.
- Zero resultant
- The forces are balanced.
- Non-zero resultant
- The forces are unbalanced.
Section 4 of 5Several pairs of forces
- Boat
- The forces are thrust, water resistance, air resistance, weight and upthrust.
- Weight is balanced by upthrust, so the boat does not move up or down.
- Thrust is balanced by water and air resistance, so the boat moves forward at a steady speed.
- Stationary or steady speed
- The forces on the object are balanced.
- Speeding up, slowing down or changing direction
- The forces on the object are unbalanced.
Section 5 of 5Do you need a force to keep moving?
- Common idea
- Many people think that a moving object, such as a leaf falling at a steady speed or a rolling football, must have a force acting on it.
- Correct idea
- An object can move without a resultant force.
- A leaf falling at a steady speed has weight balanced by air resistance.
- Without friction and air resistance a ball would carry on at a steady speed forever.
- Galileo had this idea, and it became part of Newton's first law.
- Ball in motion
- While in contact with a player's foot the forces are unbalanced and it accelerates.
- After leaving the foot, air resistance and friction are unbalanced and it decelerates.
- If forces balance, it continues at the same speed.
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