1. FORCE, MOTION AND ENERGY
TODAY’S OUTCOMES:
FORCE, MOTION AND ENERGY
- Review concepts of weight, mass, and density
- Investigate the relationship between force and acceleration
- Extend the concept of balanced forces
to moving objects with constant velocity

2. Density = Mass / Volume ⇒ Density × Volume = Mass
> You can buy an inflatable children’s wading pool that is a square 2 m x 2m. If you
put 20 cm of water in it, how many kilograms have you added to your deck? What does
this much water weigh?
Density = Mass / Volume ⇒ Density × Volume = Mass
Density of water = 1000 kg/m3
Volume of the pool = 2 m × 2 m × 0.20 m
= 0.8 m3
Mass = 1000 kg/m3 × 0.8 m3 = 800 kg
From the table of weights and masses, we see 1 Newton is the weight of approximately 100 grams of mass.
800 kg = 800,000 g, which has a weight of approximately 8000 N.

3. > Submarines dive and rise underwater through the use of ballast tanks. When these tanks are filled with water, the submarine sinks. When the submarine rises, it pumps air into the tanks and pushes the water out of the tanks. Explain how the buoyancy of the submarine relates to
a) the average density of the submarine when the ballast tanks are filled with air or water.
b) the effects of buoyant force on the submarine due to the amount of water its volume displaces.
The overall density of the submarine is increased when the tanks are filled with water -
when this exceeds 1 g/cm3 it will sink. When the water is pumped out, the density decreases, causing it to rise.
Another way to look at this is to see that the volume of the part of the submarine that contains air decreases when water is pumped in, and the buoyant force decreases.

4. Buoyant Force Analogy between Atwood Machine and Buoyancy
scale: +1.2 N
scale: +1 N
+0.8 N
+1 N
–2 N
–2 N
The force of buoyancy equals the weight of the
water displaced by an object’s volume.
If 2 objects have equal volumes, they have
equal buoyant force!
More volume = more buoyant force

5. Buoyant Force The weight of steel or brass underwater isn’t much
different than out of water why?
vs.
WHY?
Which has higher density?
Which displaces more water: 1 g of rock,
or 1 g of steel?
A lower density has more volume per gram, so a gram of rock displaces more water than a gram of steel -
ROCK HAS MORE BUOYANT FORCE PER GRAM

6. WHAT YOU ARE EXPECTED TO KNOW:
- Definitions of force, weight, mass and density
- How density can be used to predict mass or volume
- The determination of buoyant force from the weight of displaced water
WHAT YOU ARE EXPECTED TO KNOW:

7. FORCE, MOTION AND ENERGY
TODAY’S OUTCOMES:
FORCE, MOTION AND ENERGY
- Review concepts of weight, mass, and density✓
- Investigate the relationship between force and acceleration
- Extend the concept of balanced forces
to moving objects with constant velocity

8. QUICK NOTE ON CONVERTING
VOLUMES
1 m = 100 cm
1 m2 = 1 m × 1 m = 100 cm × 100 cm = 10,000 cm2
1 m3 = 1 m × 1 m × 1 m = 100 cm × 100 cm × 100 cm
= 1,000,000 cm3
Density of water is 1000 kg/m3
1000 kg/m3 = × 1 kg/1 m3
= × g / 1,000,000 cm3
= 1 g/cm3
1 kg = 1000 g
1 m3 = 1,000,000 cm3