2.8 gravity

Chapter 2 Forces and Motion

2.8 Gravity

2.8 Gravity

2.8 GravityAccording to Newton, objects fall because they are pulled towards the Earth by the

force of gravity.

2.8 GravityThe pull of gravitational force;

(a)keeps things on the earth. (b)brings things down to earth when

they are thrown upwards (c)holds the moon in its orbit round

the Earth (d)captures returning space capsules

and pulls them into orbit.

2.8 GravityThe pull of gravity causes objects

to fall with acceleration.

This means that objects that fall are moving with increasing velocity.

2.8 GravityThe magnitude of the acceleration

due to gravity depends on the strength of the gravitational field.

The gravitational field is the region around the Earth in which an object experiences a force towards the centre of the Earth.

Earth’s gravitational field strength, g, is about 9.8 N kg-1

2.8 GravityFree Fall

An object is falling freely when it is falling

under the force of gravity only.

(No air resistance)

2.8 Gravity

Free fall in vacuum

Not a free fall in air due to

air resistance

2.8 Gravity

• When an object falls, g = 9.8 m s-2

• When an object is thrown upwards,

g = - 9.8 m s-2

• At the highest point, v = 0

2.8 GravityObjects dropped under the influence of the

pull of gravity accelerate at a constant rate.

This acceleration is known as the gravitational acceleration, g.

The value of g is 9.8 m s-2. In calculation, the value of g is often taken to be 10 m s-2

for simplicity. The acceleration due to gravity does not

depend on the mass and shape of the falling object.

All objects falling freely with the same acceleration.

2.8 Gravity

Weight and Mass

2.8 GravityWeight (W) and Mass

(m)

2.8 GravityWeight (W) and Mass

(m)

2.8 GravityWeight (W) and Mass

(m)

2.8 GravityWeight (W) and Mass

(m)

2.8 GravityWeight (W) and Mass

(m)

2.8 GravityWeight (W) and Mass

(m)

2.8 GravityWeight (W) and Mass

(m)

Your weight in an elevator!!!

2.8 GravityWeight (W) and Mass

(m)Your mass = 50 kg

2.8 GravityWeight (W) and Mass

(m)Your mass = 50 kg

Acceleration = a

You feel HEAVIER!

2.8 GravityWeight (W) and Mass

(m)Your mass = 50 kg

2.8 GravityWeight (W) and Mass

(m)Your mass = 50 kg

You feel LIGHTER!

2.8 GravityWeight (W) and Mass

(m)

http://www.youtube.com/watch?v=7tEkKhMVoS4

• http://www.batesville.k12.in.us/physics/phynet/mechanics/newton2/ElevatorProblem.html

• http://education-portal.com/academy/lesson/newtons-laws-and-weight-mass-gravity.html#lesson

1. Subra is standing on a weighing scale inside an elevator. If Subra’s mass is 63kg, find the reading of the weighing scale when the lift,

a. stationaryb. moving upward with a constant velocity,

15 ms-1.,c. moving upward with a constant acceleration,

1 ms-2.d. moving downward with a constant

acceleration, 2 ms-2.[use g = 10 m/s^2]

(a) W = mg = 63(10) = 630 N(b) W = mg = 63(10) = 630 N(c) R = mg + ma = 63(10)+63(1) = 693 N(d) R = mg - ma = 63(10)-63(2) = 504 N