Force and Motion

11.1 Motion

• Motion: Described as fast or slow in daily life, but these descriptions are subjective.

• Speed, Distance, and Time: Used to precisely describe motion in science.

  • Speed: How fast an object moves; the higher the speed, the faster the object moves.

  • Distance: measured in meters (m)

  • Time: measured in seconds (s)

  • Speed: measured in meters per second (m/s or $ms^{-1}$)

Average Speed

• If the speed of an object varies, average speed can describe how fast the object moves over the whole journey.

• $Average Speed = \frac{distance}{time}$

• Example: Runner P runs a longer distance than Runner Q in the same amount of time, so P is faster.

Speed Signs

• Show the upper speed limit for vehicles, typically in kilometers per hour (km/h or $km h^{-1}$).

• $1 \frac{km}{h} = \frac{1 km}{1 h} = \frac{1000 m}{60 \times 60 s} = 0.278 \frac{m}{s}$

• Speed limit in Hong Kong: Generally 50 km/h unless otherwise specified.

• Average walking speed of a human: 1.4 m/s.

Distance-Time Graph

• Records the distance traveled by an object at different moments in time.

Calculating Average Speeds

• Using a distance-time graph to calculate average speeds of runners P and Q.

• Runner P: $Average Speed = \frac{30}{5} = 6 \frac{m}{s}$

• Runner Q: $Average Speed = \frac{20}{5} = 4 \frac{m}{s}$

Interpreting Distance-Time Graphs

• A distance-time graph records the distance an object has traveled at different times.

• Example: A man travels from end A to end B of a bridge. Analyzing the graph provides distance and time data.

Analyzing Motion from Distance-Time Graph

• Horizontal Line: Object is at rest.

• Slanted Straight Line: Object is moving at a constant speed.

• Steeper Line: Higher average speed.

Uniform Motion and Non-Uniform Motion

• Uniform Motion: Object moves at a constant speed in one direction; distance-time graph is a straight line.
  *Example: A train moving along a straight section of track at a constant speed.

• Non-Uniform Motion: The speed and/or direction of the object changes; distance-time graph is not a straight line.
  *Example: An MTR train moving between two stations, where it speeds up leaving one station and slows down approaching the next.

Practical Application: Motion of a Glider on an Air Track

• Demonstrates uniform and non-uniform motions using a glider and air track.

Examples of Motion

• Uniform Motion: Motion of an electric roller shutter when raised or lowered.

• Non-Uniform Motion: Motion of a bus in the city, a roller coaster, or people on a travelator.

Section Summary 11.1

• Average speed formula.

• Distance-time graph interpretation.

• Uniform versus non-uniform motion.

11.2 Effects of Forces and Ways to Describe Forces

Effects of Forces

• A force is a push or pull on an object.

• Effects of forces:

  • Changing the speed of an object (starting, stopping, speeding up, slowing down).

  • Changing the direction of motion of an object.

  • Changing the shape of an object.

Measurement of Force

• The unit of force is the newton (N).

• Spring balance: Measures pulling force.

• Force sensor: Measures both pulling and pushing forces, used with data logging apps. *The force of holding an orange is about 3N *Forces can Change the Shape of an Object

  • Kneading Dough
    *Squeezing a Spring

Representation of Force

• A straight arrow represents a force in a diagram.

  • Direction of the arrow: Indicates the direction of the force.

  • Length of the arrow: Indicates the size of the force.

Skill Builder 11.1: Drawing Forces

• Steps to drawing an arrow representing a force.

• Example: Drawing the holding force on a gift box.

Section Summary 11.2

• Effects of force, instruments for measuring force, unit of force, and representation of force in diagrams.

11.3 Balanced Forces and Free-Body Diagrams

Types of Forces

• Forces are classified as:

  • Contact Forces: Direct contact between objects (e.g., pulling, friction).

  • Non-Contact Forces: Act at a distance (e.g., magnetic forces, electrostatic forces, force of gravity).

Balanced Forces and Unbalanced Forces

• Unbalanced Force: When a force acts on a trolley, its motion changes.

• Balanced Forces: Two forces of equal size in opposite directions cancel each other out; the object does not move.

Net Force

• The overall force acting on an object.

Balanced Force Definition

• When all forces are balanced (net force is zero), their effects are canceled out, keeping the object's speed and move direction unchanged. If the object is at a rest originally, it will remain still, otherwise, it will move at the original speed in the original direction

Impact of unbalanced forces

• If the forces acting on an object are not balanced (net force is not zero), the speed and/or the direction of motion of the object will change

Big Science Ideas

• Change and constancy in force and motion.

• If all the forces acting on an object are balanced, the object will stay at rest or move in uniform motion.

• If the forces acting on an object are unbalanced, the object will move in non-uniform motion.

Balanced vs Unbalanced Forces Examples

• Examples: Sledge pulled at a constant speed, a lamp staying at rest (balanced forces), rope being pulled (unbalanced forces).

Free-Body Diagrams

• A tool to find out whether the forces acting on the object are balanced or not.
  *Diagram shows all the forces acting on the object only, but not those exerted by the object.

Skill Builder 11.2: Drawing Free-Body Diagrams

• Steps to drawing a free-body diagram (e.g., a hedgehog rolling into a ball).

Section Summary 11.3

• Force definitions, balanced vs. unbalanced forces, and free-body diagrams.

11.4 Gravity

What is Gravity?

• The non-contact attractive force between an object and the Earth.

• All objects on Earth are subject to the force of gravity.
  *Formula: F = ma

Free Fall

• When an object falls under gravity with no other forces acting on it.

• Free fall is a non-uniform motion (object falls faster and faster).

Mass and Weight

• Mass: The amount of matter in an object, measured in kilograms (kg) or grams (g).

• Weight: The force of gravity acting on the object, measured in newtons (N).

• Larger mass experiences a larger force of gravity.

• The mass of an object is fixed everywhere, but the weight may change.

weight change

• The mass remains unchanged, but the weight may vary from pace to space.
  *Note that the force arrow of weight is usually drawn from the centreof the object.

Discussion on Mass and Weight

• The force of gravity between two objects increases as the mass of either object increases.

• On Earh, the ratio between the weight and mass of an object is fixed, and it's around 10

• $\frac{Weight}{Mass} = 10$

Section Summary 11.4

• Force of gravity, free fall, mass and weight definitions and relationships.

11.5 Friction and Air Resistance

Friction

• The force opposing the push when trying to slide an object over a surface.

• Arises when two surfaces in contact slide over each other.

• It is a contact force, and its direction is opposite to the moving direction of the object.
  *Larger friction is caused on a rough surface

measuring friction

• Before the block starts to move, the pulling force is the same size as the friction acting on the block by the bench.

Other effects of Friction

• Heating effect (rubbing hands together).

• Wear (objects moving over each other may wear down).

Air Resistance

• The upward force acting on objects falling steadily in the air.

• Occurs when an object moves in the air.

• A contact force that opposes the motion of the object.

• Increases with the speed of the object.
  *Air resistance becomes larger as the speed of the object increases
  Like friction, air resistance also produces heating effect. This effect becomes very strong when the object is moving at very high speed.

Reducing Friction and Air Resistance

• Friction and air resistance can slow down or stop an object; people use different ways to reduce these forces.

• The streamlined suit and helmet* worn by cyclist Lee Wai-sze at the Tokyo Olympic Games were results of scientific research. The suit, named Aero Speedsuit, can save more power than regular bike suits. Wind tunnel tests were carried out to find out the best riding posture and to develop the best material for the suit to reduce air resistance.

Air density reduce air resistance

• Reduce the amount of air around an object can minimize the effect of air resistance on the object.

Useful Effects of Friction and Air Resistance

• Friction and air resistance are essential in our life (e.g., prevent slipping, slow down motion, make rolling easier).

Parachute Mechanism

• A parachute* slows down a falling object. In Practical 11.10, we will investigate the factors affecting the falling speed of a parachute.

Section Summary 11.5

• Summarizes the key points about friction and air resistance, their effects, and how to reduce them.

11.6 Action and Reaction

What are Action and Reaction?

• Forces always occur in pairs.
  *When a person hits the brick on the hand, he exerts a force on the brick and the brick also exerts a force on him at the same time to make him feel pain.

Plunger Trolley Concept

*As a result, the two trolleys move apart from each other (Fig 11.31). These two forces form an action-and-reaction pair. Note that the paired forces act on different objects and in opposite directions.

Jet Pack Mechanism

*A real jet pack makes use of action-and- reaction pair to provide the lifting force. Let us use a blown-up balloon to explain this. The balloon flies off when it is let go.

Relation about Sizes of Action and Reaction

• Action and reaction always equal in size.

• Forces always occur in action and reaction pairs. Action and reaction act on different objects that interact with each other;
  are in opposite direction;
  are equal in size.

Examples of Action-and-Reaction Pairs

• Examples: A swimmer pushing off a wall, a person leaning on a wall, hitting a tennis ball, Earth exerting force on an object.

Action and Reaction pair

• always occur in opposite directions

• acting on distinct objects

• equal in terms of size

Section Summary 11.6

• Forces always occur in pairs called action and reaction; their properties are outlined.

11.7 Space Flight

Flying up into Space

• To lift off from Earth and escape to outer space, an object must overcome gravity.

• Space tourism as an extension.

• The gases exert an upward force of equal size on the rocket at the same time, inside therocket, fuel burns to produce a lot of hot gases.
  These two forces form an action-and-reaction pair

Space Rockets

• Space rockets are long and narrow (Fig 11.37). A streamlined-shape design can minimize the air resistance when the rockets are flying in Earth's atmosphere at a high speed.
  *A rocket needs to fly to space, meaning that they need to escape the surface gravity.

Moving in Space

• No air, hence no air resistance.

• Devices released from the rocket move in frictionless motion.
  *The devices will not slow down even if they get released from the rocket; meaning that they go into a frictinless state
  There is micro-gravity motion where astronauts and objects float around, giving the sensation of weightlessness.

Micro-Gravity examples

*Astronauts can post in a funny way
*Hair stands on ends

Daily life considerations for space mission

*Search and watch videos about daily life and fun experiments under micro-gravity on the Internet. Then try to design an experiment to be carried out under micro-gravity conditions. You may take the experience of some Hong Kong students by watching a news report interviewing them on the Internet.

Returning from Space

• High temperature and high speed are two main problems.

Heat Shield and Parachute Function

• Spacecraft has a at its base to prevent the cabin from overheating when it returns to the Earth from space. • The slows down the spacecraft by increasing the air resistance.

Section Summary 11.7

• Space flight, rockets, moving in space, and returning from space.