FORCE and MOTION Unit 1

Laws of motion LESSON 1

Newton’s Law of Motion

1st Law of Motion An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This law is often called "the law of inertia". Inertia comes from mass of the object.

RESULTANT FORCE An object may have several different forces acting on it, which can have different strengths and directions. But they can be added together to give the resultant force.

BALANCED & UNBALANCED FORCE If two individual forces are of equal magnitude and opposite direction, then the forces are said to be balanced. An object is said to be acted upon by an unbalanced force only when there is an individual force that is not being balanced by a force of equal magnitude and in the opposite direction.

Law of Inertia The property that a body has that resists motion if at rest, or resists speeding or slowing up, if in motion, is called inertia. Inertia is proportional to a body's mass, or the amount of matter that a body has. The more mass a body has, the more inertia it has.

2nd Law of Motion The Second Law of Motion states that if an unbalanced force acts on a body, that body will experience acceleration ( or deceleration), that is, a change of speed. Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).

One can say that a body at rest is considered to have zero speed, ( a constant speed). So any force that causes a body to move is an unbalanced force. Also, any force, such as friction, or gravity, that causes a body to slow down or speed up, is an unbalanced force. This law can be shown by the following formula F= ma F is the unbalanced force m is the object's mass a is the acceleration that the force causes

Example of Newton’s 2nd Law of Motion Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s² Using Newton's Second Law, you can compute how much force Mike is applying to the car. Effect of Resultant Force

Acceleration Accelerating is when a moving object speeds up. Decelerating is when a moving object slows down. Acceleration is the rate of change of velocity. It measures how quickly the velocity of an object is changing over time. It is a vector quantity.

Example 1

Example 2

Example 3

Example 4

Solution

Weight When we drop an object, the object accelerates towards the ground. This indicates that there is a downward force acting on the object. This force is the weight of the object. The weight of an object is the force of gravity acting on the object. Since weight is also a force, we can apply Newton’s Second Law of Motion to get weight. w = mg Where w is weight in N, m is mass in kg and g is gravitational acceleration in N/kg or m/s². Gravitational acceleration is the acceleration that an object experiences due to the force of gravity. On the surface of the Earth, all objects fall with an acceleration of 9.8 m/s² assuming that it is falling under the force of gravity only.

Gravitational Acceleration

Example 5

Test Yourself

3rd Law of Motion For every action there is an equal and opposite re-action. This means that for every force there is a reaction force that is equal in size, but opposite in direction. That is to say that whenever an object pushes another object it gets pushed back in the opposite direction equally hard. Action-Reaction Forces – pair of forces at work and neither force exists without the other.

Why does the vase does not drop to the ground?

Test Yourself What is Newton’s Third Law of Motion? When a boy on skates pushes against a wall, what happens to him? Explain why that happens.

Don’t Print this!