Unit Two: Dynamics Part 2: Weight vs Mass

Force – Quick Revisit, Already covered in this unit Symbol: F Formula: F=ma Force = mass x acceleration Units: kg x m/s 2 = Newtons (N)

Gravitational Forces – Quick Revisit (already covered in your table) Symbol: Fg We typically look at Fg as a a non-contact force caused by the pull of the Earth’s centre on an object In reality, gravity acts as an attractive force between any two objects. The force is proportional to the objects so since Earth is so much larger than most objects we look at, we only think about it as Earth’s pull.

Gravitational Forces Example to think about: Consider the following information and then compare the gravitational force on the SAME OBJECT in each case. THINK ABOUT IT and discuss with your neighbours!!!(5 min) A man standing near the equator (distance from Earth’s centre = 6378 km) A man standing near the North pole (distance from Earth’s centre = 6357 km) A man standing in the International Space Station (distance = 6628 km) A man in a space ship past Pluto

Gravitational Forces Consider the following information and then compare the gravitational force on the SAME OBJECT in each case. The man standing closest to the CENTRE of Earth will have the most gravitational force because of its closeness to the Earth. Least to most Fg: Pluto Space Station Equator North Pole Even though we consider the force of gravity to be the same on the Earth, it does differ slightly depending on how far we are from the centre. The Earth is not a perfect sphere so there is more gravity at the North/South Poles than at the Equator (but not a big difference).

Gravitational Forces (copy) Gravitational force decreases as we increase how far we are from the centre of the Earth ****ADD THIS INFORMATION INTO YOUR TABLE OF FORCES (copied before the break)!

Weight Vs. Mass (copy) Weight and mass are NOT THE SAME. Mass = the quantity of matter an object contains. Mass for the same object is constant. It is measured in kg.

Weight (copy) Weight = the force of gravity acting on a mass. Weight can change. It is measured in Newtons. Weight = mass x gravitational force F g = mg (see Formula Sheet) We can use a spring scale (Newtonmeter) to measure *** Usually gravitational force is 9.81m/s 2

Weight Can Change… page in your text book and on formula sheet What do you notice? Why do you think this is?

Copy Weight can change depending on the gravitational pull of the planet you are on. Notice there is more acceleration due to gravity (g) on Jupiter than on Earth. This is due to the size of Jupiter.

Examples of Weight Problems Example 1: Mrs. Evans’ dog Pi has a mass of 23.5kg. What would Pi’s weight be: A) On Earth? B) On Jupiter (where g = 25.9 m/s 2 ) C) On the Moon (where g = 1.64 m/s 2 ) D) What is Pi’s mass on the Moon?

Examples of Weight Problems Example 1: Mrs. Evans’ dog Pi has a mass of 23.5kg. What would Pi’s weight be: A) On Earth? Fg = mg Fg = (23.5)(9.81) Fg = = 231 N

Examples of Weight Problems Example 1: Mrs. Evans’ dog Pi has a mass of 23.5kg. What would Pi’s weight be: B) On Jupiter (where g = 25.9 m/s 2 ) Fg = mg Fg = (23.5)(25.9) Fg = = 609 N

Examples of Weight Problems Example 1: Mrs. Evans’ dog Pi has a mass of 23.5kg. What would Pi’s weight be: C) On the Moon (where g = 1.64 m/s 2 ) Fg = mg Fg = (23.5)(1.64) Fg = = 38.5 N

Examples of Weight Problems Example 1: Mrs. Evans’ dog Pi has a mass of 23.5kg. What would Pi’s weight be: D) What is Pi’s mass on the Moon? 23.5kg Pi’s mass DOES NOT CHANGE!!!

Examples of Weight Problems Example 2: A student standing on a scientific spring scale on Earth finds that he weighs 825N. Find his mass. Fg = mg 825 = m(9.81) m = 825/9.81 = = 84.1 kg

Practice: Page 137, 1 to 4 Page 137, #1, 2, 3, 4