Understanding the effects of a force

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Malaysia SPM Form 4, Chapter 2: Force and Motion
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UNDER CONSTRUCTION

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1 Mind Map
2 Force
3 Effects of Force
4 Newton's Law of Motion
5 Frequenly Asked Question
6 Additional notes
7 External Links

[edit] Mind Map

[edit] Force

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

[edit] Effects of Force

When a force acts on an object, the effects can be

change of the size of the object,
change of the size of shape,
change of the stationary state,
change of the speed and
change of direction of the object.

[edit] Newton's Law of Motion

[edit] Newton's First Law

We have discuss Newton's first law in inertia. Newton's First Law states that when there is no net force acting on an object, the object is either stationary or move with constant speed in a straight line.

In other words, a stationary object will remain stationary if there is no net force acting on the object. A moving object will continue moving with the same speed in a straight line, if the net force acting on the object is zero.

[edit] Newton's Second Law

When discussing Newton's First Law, we have learned that when the net force acting on an object is zero, the object will either stationary or moving with uniform velocity. In Newton's Second Law, we will discuss the situation where the net force is not equal to zero.

According to Newton's Second Law,

The rate of change of momentum of a body is directly proportional to the net force acting on the body and is in the same direction.

Let's see what does this means.

Rate of change of momentum means how fast the momentum change. If the net force acting on an object is not zero, the net force will cause a momentum change to the object. How fast the momentum change depends on how big is the force. The greater the force, the greater the momentum change.

Momentum is defined as the product of mass and velocity:

Momentum = Mass x Velocity

Implication:

When there is resultant force acting on an object, the object will accelerate (moving faster, moving slower or change direction).

[edit] Formula of Force

From Newton's Second Law, we can derived the equation

Example 1

A box of mass 150kg is placed on a horizontal floor with a smooth surface; find the acceleration of the box when a 300N force is acting on the box horizontally.

Answer:
F = ma
(300) = (150)a
a = 2 ms^-2

Example 2

A object of mass 50kg is placed on a horizontal floor with a smooth surface. If the velocity of the object changes from stationary to 25.0 m/s in 5 seconds when is acted by a force, find the magnitude of the force that is acting?

Answer:
We know that we can find the magnitude of a force by using the formula F = ma. The mass m is already given in the question, but the acceleration is not give directly.

We can determine the acceleration from the formula

$\begin{array}{l} a = \dfrac{{v - u}}{t} \\ a = \dfrac{{25 - 0}}{5} = 5ms^{ - 2} \\ \end{array}$