1. A force is push or pull exerted on an object.
2. Force is a vector quantity that has magnitude and direction.
3. The unit of force is Newton ( or kgms^-2).

Unbalanced Force/ Resultant Force

When the forces acting on an object are not balanced, there must be a net force acting on it. The net force is known as the unbalanced force or the resultant force.

When a force acts on an object, the effect can change the

1. size,
2. shape,
3. stationary state,
4. speed and
5. direction of the object.

Types of Forces

Friction

1. Friction is the force that oppose motion.
2. There are 2 types of friction that you need to know in SPM:
1. Static friction - friction between 2 surfaces that are not moving relative to each other
2. sliding friction - friction where an object slides or rubs against another surface.
3. Video below discuss the differences between these 2 types of friction.

Tension

When a rope attached to an object is pulling on the object, the rope exerts a force T on the object, and the magnitude T of that force is called the tension in the rope.

Weight (Gravitational Force)

1. Weight is the gravitational force acted on an object.
2. It is always point towards the centre of the earth.

Normal Reaction (Normal Contact Force)

Normal reaction is a force acting perpendicular to two surfaces in contact with each other.

 

Rocket

1. Mixture of hydrogen and oxygen fuels burn in the combustion chamber.
2. Hot gases are expelled through the exhausts at very high speed .
3. The high-speed hot gas produce a high momentum backwards.
4. By conservation of momentum, an equal and opposite momentum is produced and acted on the rocket, pushing the rocket upwards.

Jet Engine

1. Air is taken in from the front and is compressed by the compressor.
2. Fuel is injected and burnt with the compressed air in the combustion chamber.
3. The hot gas is forced through the engine to turn the turbine blade, which turns the compressor.
4. High-speed hot gases are ejected from the back with high momentum.
5. This produces an equal and opposite momentum to push the jet plane forward.

 

Before explosion both object stick together and at rest.	After collision, both object move at opposite direction.
Total Momentum before collision Is zero	Total Momentum after collision : m1v1 + m2v2
From the law of conservation of momentum: Total Momentum Before collision = Total Momentum after collision 0 = m1v1 + m2v2 m1v1 = - m2v2 (-ve sign means opposite direction)

 

Formula

Elastic and Inelastic Collision

 

Formula:

 

When the handle of the hammer hit on the surface,  the handle stop but the head of the hammer keep on moving downward, owing to the effect of inertia. As a result, hammer head will fit tighter into the hammer.

Japanese sumo wrestlers normally have great body mass. This is because the greater mass can increases the effect of inertia and hence more difficult to be moved by their opponents.

When a prey trying to run off from its predator, it will run in zig zag rather than a straight path.

Explanation:
A predator usually has greater mass and hence greater effect of inertia. It is more difficult for the predator to change direction compare to the prey.

Animals shake their body to dry their fur.
We can spin an umbrella to remove water on the umbrella.

Explanation:
When the direction (of the fur/ umbrella) change, the water droplets keep on moving forward due to the effect of inertia, and hence leave the fur of the animals/umbrella.

Ways to Reduce the Negative Effect of Inertia

The tank in a tanker lorry is divided into smaller tanks. This may reduce the motion of the liquid inside the tank and hence reduces the effect of inertia.


Seat belt and Airbag
The passengers in a car will be thrown forward when the car stop suddenly due to the effect of inertia. The seat belt and airbag can reduce the impulsive force acted on the body of the passenger and hence prevent serious injury.

 

Jerking a Card

When the cardboard is jerked quickly, the coin will fall into the glass.

Explanation:

• The inertia of the coin resists the change of its initial state, which is stationary.
• As a result, the coin does not move with the cardboard and falls into the glass because of gravity.

Pulling a Book

When the book is pulled out, the books on top will fall downwards.
Explanation:

• Inertia tries to oppose the change to the stationary situation, that is, when the book is pulled out, the books on top do not follow suit.

Pulling a Thread

1. Pull slowly - Thread A will snap.
Explanation:

• Tension of thread A is higher than string B.
• Tension at A = Weight of the load + Pulling Force

2. Yank quickly - Thread B will snap.
Explanation:

• The inertia of the load prevents the force from being transmitted to thread A, hence causing thread B to snap.

Passengers in a Vehicle

Phenomenon 1

When a car stop, passengers in the car will be thrown forward.

Explanation
When a car stops, the effect of inertia causing the passengers' body maintain their motion forward. As a result, the passengers are thrown forward.

Phenomenon 2

When a car accelerates, passengers in the car will be thrown backward.

Explanation
When a car accelerates and moves faster, the body of the passenger tends to maintain its state of motion due to the effect of inertia. As a result, the passenger is thrown backward as he moves slower than the car.

Larger Mass - Greater Inertia

Bucket filled with sand is more difficult to be moved. It's also more difficult to be stopped from swinging.

Explanation:

• Object with more mass offers a greater resistance to change from its state of motion.
• Object with larger mass has larger inertia to resist the attempt to change the state of motion.

Empty Cart is Easier to be Moved

An empty cart is easier to be moved compare with a cart full with load. This is because a cart with larger mass has larger inertia to resist the attempt to change the state of motion.

Car with Higher Load is More Difficult to be Controlled

A car loaded with more passengers is more difficult to be controlled.

Explanation:
This is because, with more passenger, the mass increases, and hence increase the effect of inertia. This make the car  more difficult to change its speed, more difficult to stop,  and more difficult to change direction.

 

Analysing Velocity-Time Graph

1. When analysing velocity-time graph, always keep in mind that the gradient of the graph is equal to the acceleration of the graph.
2. If the gradient is constant, then the acceleration is constant. If the gradient increase, then the acceleration increase and etc.
3. Table below shows a few cases of velocity-time graph for different types of motion.

Uniform velocity

Uniform acceleration

Increasing acceleration

Uniform deceleration

Decreasing acceleration

Converting a Velocity-Time graph to Acceleration-Time graph

1. In order to convert a velocity-time graph to acceleration time graph, we need to find the gradient of the velocity time graph and plot it in the acceleration-time graph.
2. The 5th and 6th videos below explain how to sketch an acceleration-time graph form a given velocity-time graph.

Free Falling

1. Free falling is a motion under gravitational force as the only force acting on the moving object.
2. The acceleration of a free falling object is always constant.
3. On the surface of the earth, the acceleration of is equal to 10ms^-2, and is named as gravitational acceleration.
4. In SPM, you need to know the graphs of free falling of the following movement
   1. Launching object upward.
   2. Dropping Object from High Place
   3. Object Falling and Bounce Back

Launching Object Upward

1. When you launch an object upward, its velocity decreases at a constant rate, hence it's a straight line with negative gradient in a velocity-time graph. 
2. Since the velocity decreases at a constant rate, hence the acceleration is constant. Also, the acceleration is negative because the speed decreases in positive direction. Therefore, the acceleration graph is a horizontal line in the negative domain.

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1. When an object drops from a high place, its velocity increases at a constant rate, hence it's a straight line with positive gradient in a velocity-time graph. 
2. Since the velocity increases at a constant rate and the speed increases in positive direction hence the acceleration is constant and positive.

 

1. There are 3 types of motion graph, namely
   1. the displacement-time graph
   2. the velocity-time graph
   3. the acceleration-time graph.
2. When analysing a graph, it's important for us to know what's the physical quantity that's represented by the gradient of the graph and the area below the graph. 
3. For example, in a displacement-time graph, the gradient represent the velocity of the moving object, whereas in a velocity-time graph, the gradient represent the acceleration of the moving object.
4. It's also important for you to know how to find the gradient of a straight line from a graph.
5. Sometime, you will be asked to convert a displacement-time graph to a velocity-time graph or convert a velocity-time graph to an acceleration-time graph.

Analysing Displacement - Time Graph

1. When analysing displacement-time graph, always remember that the gradient of the graph represents the velocity of the graph.
2. Therefore, if the gradient of the graph is positive, the velocity is positive, and if the gradient of the graph is negative, the velocity is negative.
3. A negative velocity indicates that the object moves in opposite direction.
4. Table below shows the displacement-time graph of various motion.

Velocity = 0
This is a horizontal straight line, hence the gradient = 0.
Therefore, the velocity = gradient of the graph = 0, which means the object is stationary (does not move).

Uniform Velocity

The graph is a non-horizontal straight line, hence the gradient is not equal to 0. For a straight line, the gradient is constant, hence, the velocity of the moving object is uniform.

Negative Uniform Velocity
The graph is a non-horizontal straight line, with negative gradient. For straight line, the gradient must be constant. The negative value of gradient indicates that the object moves in opposite direction.
Therefore, this graph represents a motion with uniform velocity in opposite direction.

Increasing Velocity
The graph is a curve, shows that the gradient is not constant. The gradient increases over time, indicates that the velocity increases over time.

Decreasing Acceleration
The gradient decreases over time, shows that the velocity of the moving object decreases over time.

 

Finding Acceleration

Acceleration of a motion can be determined by using ticker tape through the following equation:

Caution!:
t is time taken from the initial velocity to the final velocity.