Force

Unit of Force0Force is measured in Newtons (N)01 Newton (N) is the amount of

force required to accelerate a 1 kg object at a rate of 1 m/s2

Newton’s Three Laws

0 I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

0 II. The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma.

0 III. For every action there is an equal and opposite reaction.

What forces can do to an object01. Can cause an object to start

moving or increase the speed of an already moving object. This is called Acceleration.

02. Can cause an object to stop moving or slow down the speed of a moving object. This is called Deceleration.

03. Can cause a moving object to change direction.

Types of Forces

1. Gravitational2. Electromagnetic3. Frictional4. Strong and Weak Nuclear

Gravitational Forces

0Force of attraction between all objects as a result of their masses and distances between them.

0The higher the masses and the shorter the distances between two objects, the greater the force of gravity.

0The smaller the masses and the larger the distances between two objects, the weaker the force of gravity.

Gravity exists between ANY two objects

Why things fall to the ground

0When we drop an object, the force of gravity is strong because the earth has a large mass and is close by. The object gets pulled to the ground.

0Near the Earth’s surface, objects accelerate to the ground at a rate of 9.8 m/s2, regardless of their masses.

0The gravitational force of the Earth decreases as you move away from the planet.

0Different planets and stars will have different forces of gravity at their surfaces due to their different masses. The bigger the object the greater the force of gravity.

0Gravitational forces also explain the Earth’s tides

Mass vs. Weight

0Mass is a measure of the quantity of matter in an object. This does not depend on where the object is.

0Weight is a measure of the gravitational force acting on an object. This DOES depend on where the object is.

Acceleration

0Acceleration = Change of velocity Time Taken

0Example – A F1 car starts at 0 m/s and gets to 27 m/s in 3 seconds, what is the F1’s acceleration?

A = 27m/s – 0m/s = 9m/s2

3 s

A useful formulaWeight = Gravitational force = Mass X gravitational field

intensityOR

w = Fg = mg or F=ma

m is mass measured in kilograms (kg)g gravitational field intensity measured in Newtons/kilogram

(N/kg); this is also known as acceleration.F is the force measured in Newtons (N)

Example0What is the

gravitational force (weight) of a 60 kg astronaut on the earth’s surface? Intensity of gravitation on earth is 9.8 N/kg

F = mgF = 60 X 9.8F = 588 N

Example

0What is the gravitational force (weight) of a 60 kg astronaut on the Moon’s surface?

F=mgF= 60 X 1.67 (force of

gravity on the Moon)F= 100.2 N