Unit Two: Dynamics Part 2: Weight vs Mass

Force – Quick Revisit, Already covered on Monday Symbol: F Formula: F=ma Force = mass x acceleration Units: kg x m/s2 = Newtons (N)

Gravitational Forces – Quick Revisit (already copied on Thursday) 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: 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 (copy) Gravitational force decreases as we increase how far we are from the centre of the Earth

Weight Vs. Mass (copy) Weight and mass are NOT THE SAME. Weight = the force of gravity acting on a mass. Weight can change. It is measured in Newtons. Weight = mass x gravitational force Fg = mg We can use a spring scale (Newtonmeter) to measure *** Usually gravitational force is 9.81m/s2 Mass = the quantity of matter an object contains. Mass for the same object is constant. It is measured in kg.

Weight Can Change… page 132-133 in your text book What do you notice? Why do you think this is?

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/s2) C) On the Moon (where g = 1.64 m/s2) D) What is Pi’s mass on the Moon?

Examples of Weight Problems A student standing on a scientific spring scale on Earth finds that he weighs 825N. Find his mass.

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