Evans SPH 3U1

Physics Grade 11

Unit 2: Dynamics

Note 11: Free Fall, Gravitational Field Strength, Mass, Weight

 

Reference: Chapter 4.1

Free Fall:

Recall free fall and terminal velocity from Note 5.

If air resistance is large, terminal velocity is reached more quickly.

Larger cross sectional area experiences more air resistance than smaller cross sectional areas.

Gravitational Field Strength,g:

It is an acceleration NOT a force

Earth's gravitational field strength is the amount of force per unit mass acting on objects in the gravitational field. (comes from g = Fg/m)

                                        g = 9.8 N/kg  (direction is always toward the centre of the earth)

Note that the value of 'g' varies over the surface of the earth because the radius of the earth changes slightly with latitude and elevation.

 

Mass: Amount of matter in an object.  It's a scalar and is constant anywhere is the universe. The ability to resist force is called inertia and mass is a measure of it.
Weight: Force of gravity on an object.  Weight is a vector and is not constant (depends on which planet and the objectís distance from the centre of the planet).
Apparent Weight: 'What you feel you weigh'. It is the magnitude of the normal force.
            Weight= 

 

Example 1: Kathy has a mass of 50. kg.

a) What is Kathy's weight on the earth?

b) Kathy's physics teacher now pulls on her ponytail directly up with a force of 10. N. What does Kathy feel her apparent weight is now?

Example 2: An 2000. kg elevator is suspended by a cable (for this problem assume the cable's mass is insignificant).

a) What is the tension in the cable when the elevator is not accelerating?

b) What is the tension in the cable when the elevator is:

        (i) accelerating up at 4.0 m/s2?

        (ii) accelerating down at 4.0 m/s2?

       (iii) accelerating down at 9.8 m/s2?

Example 3: A 50. kg person is in an elevator.

a) What is the weight of the person?

b) What is the apparent weight of the person when the elevator is:

        (i) accelerating up at 2.0 m/s2?

        (ii) accelerating down at 2.0 m/s2?

       (iii) accelerating down at 9.8 m/s2?

Pg 166 #2,4

Pg 167 #5,7,10