PhET Activity for MS-PS2-4: Gravity Force Lab

DIRECTIONS:
1. Observe the initial setup: two masses, m1 (blue) and m2 (red), are shown on a ruler.
2. Note the force values for 'force on m2 by m1' and 'force on m1 by m2' displayed at the top of the screen.
3. Locate the sliders for 'Mass 1' and 'Mass 2' in the panels at the bottom of the screen.
4. Find the 'Force Values' panel on the right side, which contains options for how the force is displayed.
Take 2-3 minutes to freely explore the simulation. Drag the figures to change the distance between the masses. Use the sliders to change the mass of each object. Observe how the force values and the arrows above the masses change. Do not change any settings in the 'Force Values' panel yet.

QUESTION: Are you acquainted with the tools in this simulation?

• Yes

• No

DIRECTIONS:
1. Click the orange 'Reset Button' in the bottom-right corner to return to the default settings.
2. Using the sliders, set 'Mass 1' to 100 kg and 'Mass 2' to 100 kg.
3. Observe the force value shown at the top of the screen.

QUESTION: If you keep Mass 1 at 100 kg and double Mass 2 from 100 kg to 200 kg, what happens to the gravitational force?

• The force is halved.
• The force is doubled.
• The force stays the same.
• The force is quadrupled.

Answer explanation

The force is doubled. Gravitational force is directly proportional to the mass of the objects. Doubling the mass of one object doubles the force between them.

DIRECTIONS:
1. Keep the settings from the previous slide: Mass 1 = 100 kg, Mass 2 = 200 kg, and the distance is 4m.

QUESTION: If you change Mass 1 from 100 kg to 300 kg, the gravitational force will ___. This shows that the force is directly ___ to the mass of each object.

(a)  

Answer explanation

triple, proportional

DIRECTIONS:
1. Click the 'Reset Button'.
2. Set 'Mass 1' to 200 kg.
3. Set 'Mass 2' to 800 kg.
4. Place the masses any distance apart.

QUESTION: Even though Mass 2 is much larger than Mass 1, the force exerted by Mass 2 on Mass 1 is (a)   the force exerted by Mass 1 on Mass 2.

• greater than
• less than
• equal to

Answer explanation

The correct answer is 'equal to'. This demonstrates Newton's Third Law of Motion; the forces are an interaction pair, always equal in magnitude and opposite in direction.

DIRECTIONS:
1. Keep the masses at 200 kg and 800 kg.
2. Drag the masses closer together and farther apart.
3. Observe the arrows (force vectors) on top of each mass.

QUESTION: What do the directions of the force vectors consistently indicate about the nature of gravity?

• The forces are repulsive, pushing the masses apart.
• The forces are attractive, pulling the masses together.
• The forces are sometimes attractive and sometimes repulsive.
• The forces act perpendicular to the line connecting the masses.

Answer explanation

The forces are attractive, pulling the masses together. The force vector on each mass always points toward the other mass, indicating an attractive force.

DIRECTIONS:
1. Click the orange 'Reset Button'.
2. Set 'Mass 1' to 500 kg and 'Mass 2' to 500 kg.
3. Record the force value shown at the top of the screen.

QUESTION: If you move Mass 2 from the 6m mark to the 4m mark, halving the distance from 4m to 2m, how does the gravitational force change?

• The force is halved (1/2 as large).
• The force is doubled (2 times larger).
• The force is quadrupled (4 times larger).
• The force is quartered (1/4 as large).

Answer explanation

The force is quadrupled. Gravitational force follows an inverse square law with distance. Halving the distance increases the force by a factor of 2 squared, or 4.

DIRECTIONS:
1. Click the orange 'Reset Button'.
2. Set 'Mass 1' to 400 kg and 'Mass 2' to 400 kg.
3. Position Mass 1 at the 2m mark and Mass 2 at the 4m mark (distance = 2m).
4. Record the force.

QUESTION: If you double the distance between the masses from 2m apart to 4m apart, the force becomes ___ of its original value. If you triple the original distance from 2m apart to 6m apart, the force becomes ___ of its original value.

(a)  

Answer explanation

one-quarter, one-ninth

DIRECTIONS:
1. Click the orange 'Reset Button'.
2. Set 'Mass 1' to 10 kg and 'Mass 2' to 10 kg.
3. Position the masses as far apart as possible: Mass 1 at the 0m mark and Mass 2 at the 10m mark.
4. Observe the very small force value in decimal notation.
5. In the 'Force Values' panel on the right, click the 'Scientific Notation' button.

QUESTION: Why is scientific notation useful for representing the gravitational force?

Answer explanation

A good answer should explain that scientific notation is a more compact and less error-prone way to write very large or very small numbers. It avoids writing long strings of zeros, making the numbers easier to read, compare, and understand in terms of magnitude by looking at the exponent.

DIRECTIONS:
1. Click the orange 'Reset Button'. You will now be in the default 'Decimal Notation' view.
2. Set 'Mass 1' to 200 kg and 'Mass 2' to 200 kg.
3. Set the distance between them to 4m (e.g., place them at the 2m and 6m marks).
4. Note the force. Let's call this 'Force A'.

QUESTION: Starting with the setup in the directions ('Force A'), if you double Mass 1 to 400 kg AND double the distance to 8m, what will happen to the force?

• The force will be quartered.
• The force will be halved.
• The force will stay the same.
• The force will be doubled.

Answer explanation

The force will be halved. Doubling one mass doubles the force, but doubling the distance quarters the force (makes it 1/4 as strong). The combined effect is 2 times 1/4, which equals 1/2 of the original force.

DIRECTIONS:
1. Click the orange Reset Button'.
2. Keep the distance between the masses constant at 4m (e.g., place them at 2m and 6m).
3. Change the masses to different values and record your observations.

QUESTION: Describe the evidence from the simulation that supports the claim that gravitational force depends on the mass of the objects. Provide at least two specific data examples from your exploration.

Answer explanation

A good answer must state a clear relationship (e.g., as mass increases, force increases) and provide specific, quantitative data from the simulation. For example: 'With distance fixed at 5m, when m1=100kg and m2=100kg, the force was 2.67 x 10^-9 N. When I increased m1 to 200kg, the force increased to 5.34 x 10^-9 N. This doubling of force when mass is doubled provides evidence that force depends on mass.'