Introduction to forces
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Lesson notes
What is a Force?
- A **force** is a push, a pull or a twist.
- Forces can change the **speed**, **direction** or **shape** of an object.
- The unit of force is the **Newton (N)**.
- Forces are measured using a **Newton meter**.
Representing Forces
- Forces are represented by **arrows** in diagrams.
- The arrow shows the **direction** the force is acting.
- The **length** of the arrow indicates the **size** of the force (longer arrow = larger force).
Resultant Force
- The **resultant force** is the overall force when all individual forces are combined, taking direction into account.
- If forces act in the **same direction**, add them together.
- If forces act in **opposite directions**, subtract the smaller from the larger.
- The resultant force has both a **size** and a **direction** (e.g., 300 N to the right).
Balanced and Unbalanced Forces
- When the resultant force is **zero**, the forces are **balanced**.
- Balanced forces mean the object is either **stationary** or moving at **constant speed**.
- When the resultant force is **non‑zero**, the forces are **unbalanced**.
- Unbalanced forces cause the object to **accelerate** in the direction of the resultant force.
Example: Forces in the Same Direction
- A crate has a 100 N force and a 200 N force both to the right.
- Resultant force = 100 N + 200 N = **300 N to the right**.
- The crate will **accelerate to the right**.
Example: Forces in Opposite Directions
- A crate has a 200 N force to the right and a 100 N force to the left.
- Resultant force = 200 N − 100 N = **100 N to the right**.
- The crate will **accelerate to the right**.
Example: Balanced Forces on a Car
- The car’s **weight** (down) and **reaction force** (up) are balanced.
- The **thrust** (forward) and **air resistance** (backward) are unbalanced.
- Resultant force = 4000 N to the right, so the car **accelerates forward**.
Forces in the same direction: resultant = 300 N to the right.
Forces in opposite directions: resultant = 100 N to the right.
Slides
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Practice questions
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1.What is a force?
Easy- AA push, a pull or a twist
- BA type of energy
- CA measure of speed
- DA unit of mass
2.What is the unit of force?
Easy- ANewton (N)
- BJoule (J)
- CWatt (W)
- DKilogram (kg)
3.Which instrument is used to measure the size of a force?
Easy- ANewton meter
- BRuler
- CBalance
- DThermometer
4.Two forces of 100 N and 200 N act in the same direction. What is the resultant force?
Medium- A300 N in the direction of the forces
- B100 N in the direction of the larger force
- C200 N in the direction of the smaller force
- D0 N
5.Two forces of 100 N and 200 N act in opposite directions. What is the resultant force?
Medium- A100 N in the direction of the 200 N force
- B300 N in the direction of the 200 N force
- C100 N in the direction of the 100 N force
- D0 N
6.If the resultant force on an object is zero, what can be said about the object's motion?
Medium- AIt is either stationary or moving at constant speed
- BIt is accelerating
- CIt is decelerating
- DIt is changing direction
7.A car has a thrust of 4000 N to the right and air resistance of 2000 N to the left. What is the resultant force?
Medium- A2000 N to the right
- B6000 N to the right
- C2000 N to the left
- D0 N
8.In a tug of war, team A pulls with 300 N left and team B pulls with 250 N right. What is the resultant force and what happens?
Hard- A50 N left; the rope moves left
- B50 N right; the rope moves right
- C550 N left; the rope moves left
- D0 N; the rope does not move
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