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.

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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?
Easy
- 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?
Easy
- 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?
Easy
- 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?
Medium
- 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

More questions

9.A boat experiences a forward force of 500 N and a backward drag of 500 N. Which statement is correct?
Medium
- AThe forces are balanced, so the boat moves at constant speed or is stationary
- BThe resultant force is 1000 N forward
- CThe boat will accelerate forward
- DThe boat will decelerate
10.In a force diagram, what does the length of an arrow represent?
Easy
- AThe size of the force
- BThe direction of the force
- CThe speed of the object
- DThe mass of the object
11.A lorry has a driving force of 8000 N and a resistive force of 6000 N. What is the resultant force and effect on motion?
Medium
- A2000 N forward; the lorry accelerates forward
- B2000 N backward; the lorry decelerates
- C14000 N forward; the lorry accelerates forward
- D0 N; the lorry moves at constant speed
12.An aeroplane in flight has thrust 30000 N forward, drag 25000 N backward, lift 80000 N upward, weight 80000 N downward. What is the resultant horizontal force?
Medium
- A5000 N forward
- B0 N
- C30000 N forward
- D5000 N backward
13.A car is moving along a straight road. The engine provides a driving force of 2000 N forward, and the total resistive force (friction and air resistance) is 1500 N backward. What is the resultant force on the car?
Medium
- A500 N forward
- B500 N backward
- C3500 N forward
- D3500 N backward
14.Which of the following is NOT an effect that a force can have on an object?
Easy
- AChange its colour
- BChange its speed
- CChange its direction
- DChange its shape
15.A box is being pushed with a force of 30 N to the left and pulled with a force of 20 N to the right. What is the resultant force on the box?
Easy
- A10 N to the left
- B10 N to the right
- C50 N to the left
- D50 N to the right
16.A spring is stretched by hanging a weight on it. The spring extends by 5 cm when a force of 10 N is applied. What force would cause an extension of 8 cm? (Assume the spring obeys Hooke's law.)
Medium
- A16 N
- B8 N
- C4 N
- D20 N
17.A student measures the force needed to pull a block across a surface at constant speed. She uses a newton meter and records 2.5 N. If the block weighs 10 N, what is the frictional force?
Medium
- A2.5 N
- B10 N
- C7.5 N
- D12.5 N
18.A rocket in space has a thrust of 5000 N forward and experiences a drag of 2000 N backward. What is the resultant force?
Easy
- A3000 N forward
- B7000 N forward
- C3000 N backward
- D7000 N backward
19.A skydiver falls at constant speed. Which statement about the forces is correct?
Medium
- AThe upward force equals the downward force.
- BThe upward force is greater than the downward force.
- CThe downward force is greater than the upward force.
- DThere are no forces acting.
20.A book rests on a table. The force of gravity pulls the book downward with a force of 5 N. What is the size of the upward force from the table?
Easy
- A5 N
- B0 N
- C10 N
- D2.5 N
21.A force diagram shows an arrow of length 4 cm representing a force of 20 N. What force would a 7 cm arrow represent?
Medium
- A35 N
- B28 N
- C14 N
- D40 N
22.A car of weight 12000 N is parked on a level road. The normal reaction force from the road is 12000 N upward. The car is then pushed with a horizontal force of 300 N but does not move. What is the frictional force acting?
Medium
- A300 N
- B0 N
- C12000 N
- D12300 N
23.Which piece of equipment is used to measure the size of a force?
Easy
- ANewton meter
- BBalance
- CRuler
- DThermometer
24.In a tug of war, team A pulls with a force of 300 N to the left and team B pulls with a force of 250 N to the right. What is the resultant force?
Easy
- A50 N to the left
- B50 N to the right
- C550 N to the left
- D550 N to the right
25.A lorry has a driving force of 8000 N forward and a resistive force of 6000 N backward. What is the resultant force and what effect does it have on the lorry's motion?
Medium
- A2000 N forward, lorry accelerates forward
- B2000 N backward, lorry accelerates backward
- C14000 N forward, lorry accelerates forward
- D2000 N forward, lorry moves at constant speed
26.A car of weight 12000 N is parked on a level road. The normal reaction force from the road is 12000 N upward. The car is then pushed with a horizontal force of 500 N. The car does not move. What is the frictional force from the road?
Medium
- A500 N
- B12000 N
- C0 N
- D500 N in the opposite direction
27.Which of the following is a force?
Easy
- AA push or a pull
- BA colour
- CA temperature
- DA shape
28.Two forces of 5 N and 3 N act in the same direction. What is the resultant force?
Easy
- A8 N in the direction of the forces
- B2 N in the direction of the larger force
- C8 N in the opposite direction
- D2 N in the opposite direction
29.Two forces of 7 N and 4 N act in opposite directions. What is the resultant force?
Easy
- A3 N in the direction of the 7 N force
- B11 N in the direction of the 7 N force
- C3 N in the direction of the 4 N force
- D11 N in the direction of the 4 N force
30.A box is pushed with a force of 50 N to the right and pulled with a force of 30 N to the left. What is the resultant force on the box?
Easy
- A20 N to the right
- B80 N to the right
- C20 N to the left
- D80 N to the left
31.A car accelerates forward. Which statement about the forces acting on the car is correct?
Medium
- AThe driving force is greater than the resistive force.
- BThe driving force is equal to the resistive force.
- CThe driving force is less than the resistive force.
- DThere is no resistive force acting on the car.
32.A student measures the force needed to pull a block at constant speed across a surface. The newton meter reads 2.5 N. The block weighs 10 N. What is the frictional force acting on the block?
Medium
- A2.5 N
- B10 N
- C7.5 N
- D12.5 N
33.A rocket in space has a thrust of 5000 N forward and experiences no other forces. What is the resultant force?
Easy
- A5000 N forward
- B0 N
- C5000 N backward
- DCannot be determined
34.A car of weight 12000 N is parked on a level road. The normal reaction force from the road is 12000 N upward. The car is then pushed with a horizontal force of 500 N but does not move. What is the frictional force acting on the car?
Medium
- A500 N
- B0 N
- C12000 N
- D12500 N
35.A car of weight 12000 N is parked on a level road. The normal reaction force from the road is 12000 N upward. The car is then pushed with a horizontal force of 500 N but does not move. What is the resultant force on the car?
Medium
- A0 N
- B500 N horizontally
- C12000 N vertically
- D12500 N diagonally
36.A student uses a newton meter to pull a wooden block across a table at constant speed. The newton meter reads 3.0 N. The block has a weight of 12 N. What is the frictional force acting on the block?
Medium
- A3.0 N
- B12 N
- C15 N
- D9.0 N
37.A spring is suspended from a clamp stand. A force of 4 N causes it to stretch by 2 cm. What extension would a force of 10 N cause, assuming the spring obeys Hooke's law?
Medium
- A5 cm
- B20 cm
- C0.8 cm
- D8 cm
38.A student measures the force needed to pull a block at constant speed across a surface. The newton meter reads 2.5 N. The block weighs 10 N. What is the coefficient of friction (ratio of frictional force to normal reaction)?
Hard
- A0.25
- B4.0
- C0.4
- D2.5
39.Which of the following is used to measure force?
Easy
- ANewton meter
- BBalance
- CRuler
- DThermometer
40.In an experiment, a student hangs masses on a spring and records the extension. Which graph shows the relationship between force and extension for a spring obeying Hooke's law?
Medium
- AA straight line through the origin
- BA curve that flattens off
- CA straight line not through the origin
- DA curve that steepens
41.Forces change the speed, direction or shape of an object.
Easy
True or false?
42.In a force diagram, the longer the arrow, the smaller the force.
Easy
True or false?
43.If two forces act in the same direction, the resultant force is found by subtracting them.
Easy
True or false?
44.If two forces act in opposite directions, the resultant force is found by subtracting one from the other.
Easy
True or false?
45.A force of 30 N to the right and a force of 10 N to the right act on an object. What is the resultant force?
Easy
- A40 N to the right
- B20 N to the right
- C40 N to the left
- D20 N to the left
46.A force of 25 N to the left and a force of 15 N to the right act on an object. What is the resultant force?
Easy
- A10 N to the left
- B10 N to the right
- C40 N to the left
- D40 N to the right
47.If the resultant force on an object is zero, the object must be stationary.
Medium
True or false?
48.A car is moving at a constant speed in a straight line. Which statement about the forces is correct?
Medium
- AThe forces are balanced
- BThe driving force is greater than the resistive force
- CThe resistive force is greater than the driving force
- DThere are no forces acting on the car
49.A spring extends by 3 cm when a force of 6 N is applied. What force would cause an extension of 9 cm? (Assume Hooke's law.)
Medium
- A18 N
- B12 N
- C9 N
- D2 N
50.Match each description to the correct term about forces.
Easy
- A push, pull or twist
- The unit of force
- Instrument to measure force
- Newton (N)
- Force
- Newton meter
51.Complete the sentence about forces.
Easy
A force is a ____, a ____ or a ____.
52.A boat experiences a forward force of 500 N and a backward drag of 500 N. What is the resultant force?
Easy
53.A boat experiences a forward force of 500 N and a backward drag of 500 N. What is the resultant force?
Easy
- A-1 N
- B0 N
- C2 N
- D1 N

Revision notes

## Introduction to Forces - A force is a **push, pull or twist**. Forces can change an object's **speed, direction or shape**. - Force is measured in **Newtons (N)** using a **Newton meter**. - Forces are represented by **arrows** in diagrams: arrow length shows force size, arrow direction shows force direction. ### Resultant Force - The **resultant force** is the overall force when combining all forces acting on an object, taking direction into account. - **Same direction**: add forces together. - Example: 100 N + 200 N = **300 N to the right** → object accelerates right. - **Opposite directions**: subtract the smaller force from the larger one. - Example: 200 N right − 100 N left = **100 N to the right**. - Always state the **direction** of the resultant force. ### Balanced Forces - If resultant force = **0 N**, forces are **balanced**. - Object will either **remain stationary** or **continue moving at constant speed**. - Example: weight and reaction force on a car are balanced; thrust and air resistance may be unbalanced. ### Unbalanced Forces - If resultant force ≠ 0 N, forces are **unbalanced**. - Object will **accelerate** in the direction of the resultant force. - Example: car with thrust 5000 N and air resistance 1000 N → resultant 4000 N to the right → car accelerates right.

Force: A push, pull or twist that can change an object's speed, direction or shape.
Newton (N): The unit of force.
Newton meter: A device used to measure force.
Resultant force: The overall force when combining all forces acting on an object, taking direction into account.
Balanced forces: Forces that result in a net force of zero; object remains stationary or moves at constant speed.
Unbalanced forces: Forces that result in a non-zero net force; object accelerates in the direction of the resultant force.

Flashcards

What is a force?

A force is a push, a pull or a twist.

What can forces change about an object?

Forces change the speed, direction or shape of an object.

What is the unit of force?

The unit of force is the Newton (N).

What instrument is used to measure force?

Forces are measured using Newton meters.

How are forces represented in diagrams?

Forces are represented by arrows showing the direction the force is acting. The longer the arrow, the larger the force.

What is the resultant force?

The resultant force is calculated by combining all forces, taking their direction into account.

How do you calculate resultant force when two forces act in the same direction?

Add the forces together. For example, 100 N and 200 N both to the right give a resultant of 300 N to the right.

How do you calculate resultant force when two forces act in opposite directions?

Subtract the smaller force from the larger force. For example, 200 N right and 100 N left give a resultant of 100 N to the right.

What does a resultant force of zero mean?

The object will either remain stationary or remain moving at the same speed. The forces are balanced.

What does it mean if forces are balanced?

All individual forces combine to make a resultant force of zero.

What happens to an object when there is a resultant force?

The object will accelerate in the direction of the resultant force.

In a tug of war, if left pull is 500 N and right pull is 500 N, are the forces balanced?

Yes, the forces are balanced because they are equal and opposite.

In a tug of war, if left pull is 500 N and right pull is 300 N, what is the resultant force?

Resultant force = 500 N - 300 N = 200 N to the left.

A lorry has thrust 4000 N right and air resistance 2000 N left. Are the forces balanced?

No, the forces are unbalanced because they are not equal.

A lorry has thrust 4000 N right and air resistance 2000 N left. What is the resultant force?

Resultant force = 4000 N - 2000 N = 2000 N to the right.

A lorry has resultant force 2000 N to the right. What will happen to the lorry?

The lorry will accelerate to the right.

An aeroplane has thrust 5000 N forward and drag 5000 N backward. Are the forces balanced?

Yes, the forces are balanced because they are equal and opposite.

An aeroplane has thrust 5000 N forward and drag 5000 N backward. What is the resultant force?

Resultant force = 0 N.

An aeroplane has resultant force 0 N. What will happen to the aeroplane?

It will remain moving at the same speed (or remain stationary if already stationary).

A boat has engine force 300 N east and water resistance 100 N west. Are the forces balanced?

No, the forces are unbalanced.

A boat has engine force 300 N east and water resistance 100 N west. What is the resultant force?

Resultant force = 300 N - 100 N = 200 N east.

A boat has resultant force 200 N east. What will happen to the boat?

The boat will accelerate east.

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