🍂🍃Physics year 10🍃🍂

Balanced forces

• Forces that are equal in size and opposite in direction

• They cancel each other out

• No change in motion or shape

• Object may be still or moving at a constant speed

• Example: A book resting on a table (gravity pulls down, table pushes up)

Unbalanced Forces

• Forces that are not equal

• Cause a change in motion (speed up, slow down, change direction)

• Example: Two dogs pull a toy — if one dog pulls harder, the toy moves

Representing the forces on an object

• Forces are shown with arrows

• Arrow length = size of force

• Arrow direction = direction of force

• If the object is not moving or not accelerating, forces are balanced

Applied Force

• A push or pull by a person, animal, or machine

• Example: Holding a soccer ball — your hand applies a force upward

• Balances gravity, so the ball doesn’t fall

Free Body Diagram

• Object is shown as a square or box

• Arrows show forces acting on it

• Helps find the net force

Balanced Forces in Free Body Diagram

• Equal length arrows in opposite directions

• Forces cancel → net force = 0

• No movement or constant speed

Support Force

• The upward force from a surface

• Balances gravity when something rests on a surface

• If object doesn’t sink → support force

Net force

The sum of the forces acting on a single object

• If the forces are balanced, the net force is zero → no change in motion.

• If the forces are unbalanced, the net force is not zero → the object will accelerate (speed up, slow down, or change direction).

• add any forces acting in the same direction and subtract any forces acting in opposite directions.

• units of net force are newtons (N).

• 10 N force right, 6 N force left → Net force = 4 N to the right

• 5 N up, 5 N down → Net force = 0 N (balanced)

Newton’s first law of motion

Law of inertia- An object will remain at rest or move with constant velocity unless acted on by a net force

Explains why things don’t change speed or direction without a force

Properties of inertia

• Inertia is not a force

• It doesn't make something move or stop

• It's a property of all objects with mass,

Moving vs stationary objects and inertia

• Inertia is the natural tendency of an object to keep doing what it's already doing

• An object at rest → stays at rest

• A moving object → keeps moving at the same speed and direction

• Inertia means an object resists changes to its motion

Velocity

Velocity = speed + direction

• A change in velocity means a change in speed, direction, or both

• Inertia keeps an object’s velocity the same unless a net force acts on it

Newton’s Second Law of Motion

• Newton’s 2nd Law: F = ma
  (Force = mass × acceleration)

Proportional Relationships

• When the force acting on an object increases, the acceleration also increases.
  → This means acceleration is directly proportional to force.
  (More force = more acceleration)

• When the mass of an object increases, the acceleration decreases (if the same force is applied).
  → This means acceleration is inversely proportional to mass.

  (More mass = less acceleration)

Force Units

• 1 Newton (N) = 1 kg × 1 m/s² (f=ma)

• Standard units:

  • Force (F) → Newtons (N)

  • Mass (m) → Kilograms (kg)

  • Acceleration (a) → Metres per second² (m/s²)

• Simulation uses:

  • millinewtons (mN) = 0.001 N

  • grams (g) = 0.001 kg

• F = ma still works if you divide both mass and force by 1000

Why More Mass Needs More Force

• More mass = more inertia

• Inertia = resistance to change in motion

• A greater force is needed to accelerate a larger mass

• So: same force → smaller object accelerates more

Newton’s Third Law

• Every action has an equal and opposite reaction.

• Forces always come in pairs.

• These pairs are called action-reaction forces.

Action-Reaction forces must be:

• equal in size

• opposite in direction

• of the same type

• acting on different objects

• e.g. a horse pulls a cart (action), the cart pulls back on the horse (reaction). They don’t cancel out because they act on different things.

Labelling forces

Each label should have the following form:

F_{force type, x on y}

For example, you hit a nail with a hammer. We could represent the force like this:

F_{applied, hammer on nail}

Action reaction forces vs balanced forces

• Equal and opposite forces only cancel out when they act on the same object.

• This is when we say that they are balanced.

• But, the pairs of forces described by the third law always act on different objects.

What is speed?

• Speed describes how far something travels in a certain amount of time.

• It does not describe whether something is "fast" or "slow".

• It's a measurement of motion.

Speed Formula

speed= distance travelled/time taken

Converting between units of speed

To convert km/h → m/s, divide by 3.6

To convert m/s → km/h, multiply by 3.6

Units of speed- option 1

Speed- m/s

Distance- m

Time- s

Units of speed- option 2

Speed- km/h

Distance- km

Time- h

Instantaneous speed

An objects speed at any instant of it’s motion.

Average speed

The average of the instantaneous speeds over the whole distance travelled.

Average speed=Distance/Time

Transposing speed equation

What does a distance–time graph show?

How far an object has travelled over time.

x axis= time

y axis= distance

gradient= speed

How to read a distance-time graph

• Upward curve (steepening) = Speeding up

• Curve flattening out = Slowing down

• Straight line = Constant speed

• Flat horizontal line = Stopped

How does the slope of a distance–time graph relate to speed?

The steeper the slope, the greater the speed.
The slope = speed.

How do we calculate average speed between two times?

• Speed=gradient

• s= rise(distance)/run(time)

• speed= d₂​−d_1/ t₂-t₁

• Change in distance/ change in time

What does a speed–time graph show?

How fast an object is moving at each moment.

How to read a speed-time graph

• Upward slope (line going up)
  → Acceleration (speed is increasing)

• Downward slope (line going down)
  → Deceleration (speed is decreasing)

• Horizontal line (flat)
  → Constant speed

• Line at zero (on the time axis)
  → Object is stopped / not moving

Distance

Total distance travelled during the motion

Displacement

Straight line distance from an object’s starting point

• has magnitude(size) and direction.

• e.g. it's 120 km by a slow and winding road between Snake Gully and Gusville. But the map shows Gusville is only 70 km east of Snake Gully.

• So, the displacement from snake gully to Gusville is 70km east

Scaler quantity

Has magnitude(size) but not direction

• Distance is a scalar quantity

• Speed is a scalar quantity, because it is measured using distance and time, which are both scalar quantities.

Vector quantity

Has magnitude(size) and direction

• Displacement is a vector quantity.

• Velocity is a vector quantity

Velocity formula

Velocity= displacement/time

e.g. A map shows Gusville is only 70 km east of Snake Gully.

If it takes a driver 2 hours to get between the towns, then their average velocity is: 70/ 2

= 35km east

What's the difference between average speed and average velocity?

Speed = uses distance
Velocity = uses displacement
They’re often different!

Positive and negative velocity

Choose a direction as positive.

• Move that way = positive velocity

• Move opposite = negative velocity

( Right and up is often positive, left and down is often negative)

What is Acceleration?

Acceleration = any change in velocity
Velocity = speed + direction

It is a vector quantity
So, acceleration happens when:

• 🚀 Speed increases

• 🐢 Speed decreases

• 🔄 Direction changes

Acceleration Formula

Acceleration= change in velocity/time taken

a= v₂-v₁/t₂-t₁

Standard units of acceleration

a= m/s/s, m/s² , or ms^-2

v= m/s or ms^-1

t=s

Positive vs Negative Acceleration

• Positive acceleration = speeding up

• Negative acceleration = slowing down (also called deceleration)
  ⚠ But if you're moving in a negative direction, negative acceleration can mean speeding up in that direction!

Velocity-Time graphs: slope

The slope (tilt) tells you the acceleration:

• Upward slope = Positive acceleration (speeding up in + direction)

• Downward slope = Negative acceleration (slowing down or speeding up in – direction)

• Flat line = Constant velocity- same speed and direction (zero acceleration)

Velocity-Time graphs- other important info

On a velocity-time graph:

• Area under the line = distance travelled

• Slope of the line = acceleration

  • Steeper = faster acceleration or deceleration

  • Flat = no acceleration

What is acceleration due to gravity?

• The same force that acts on all objects on Earth.

• Acceleration due to gravity (g) has a value of about 10 m/s/s down.

• It always acts downwards towards the centre of the Earth, no matter the direction the object is going

Why would an object have a negative acceleration even when going up?

Because gravity pulls it down the whole time,

• Slows it down going up

• Speeds it up coming down

What does a straight, sloping line mean on a velocity-time graph?

• Constant acceleration

• Negative slope = acceleration is downward

• Line goes through 0 when an object changes direction