Materials

Meter sticks with loops

Mass Hanger (50 g)

2 x Spring Scales (5 N capacity)

Masking tape

2 x Ring stands with clamps

Introduction

The term mechanical equilibrium means that an object has a net force of zero on it. We know from Newton’s first law, that when an object has a net force of zero acting on it, it will continue in a state of uniform motion. If it is at rest, it will remain at rest and if the object is moving at constant velocity, it will remain moving at constant velocity. In this activity we will explore Newton’s 1^st Law from the point of view of finding the forces necessary to produce equilibrium on an object. The apparatus we will use will consist of a meter stick with loops of thread attached, hereafter known as “the bar”, two spring scales and a weight which can be suspended from different places on the bar.

Procedure and Questions

1. Examine one the provided spring scales. Note that there are different scales on opposite sides. Record the units that each scale reads.

___________ and ___________.

2. The type of quantity that can be measured with a spring scale then is either ___________ or __________.

In the following, we will use the newton (N) scale on the spring scale. When using a spring scale to make a measurement, make sure that the scale hangs straight up and down and the tongue of the spring scale is free to move.

3. Use the spring scale to determine the weight, in newtons, of the bar. Record your result in the following space. Include units.

Weight of Bar: ___________________

4. Use the spring scale to determine the weight, in newtons, of the mass hanger. Record your result in the following space. Include units.

Weight of Mass Hanger: _____________

5. You will suspend the bar simultaneously from the two spring scales. Predict what the reading on each spring scale will be when you support the bar with two spring scales. Record your prediction below. Include units.

Prediction: Scale 1:__________________ Scale 2:__________________

For the following questions, you will be taking readings from each spring scale. Use masking tape to label one of the scales as #1 and the other as #2. We will refer to readings from scale 1 as F₁ and readings from scale 2 as F₂.

6. Suspend the bar on the side with two hooks from the two scales and record the readings from both scales. Include units.

F₁ : _____________________________

F₂ : _____________________________

7. Was you prediction correct?

8. Add F₁ + F₂.

F₁ + F₂ : _____________________________

9. How does the sum of the scale readings compare to the weight of the bar?

10. Add the measured weights of the bar and mass hanger? Include units.

Combined weight of bar and hanger: ______________________________

11. Predict how the readings on the scales will change (compared to the bar only) if the mass hanger is suspended from the center of the bar.

Prediction: F₁ = ___________F₂ = ______________

12. Suspend the mass hanger from the hook in the center and record the readings of both spring scales.

F₁ : _____________________________

F₂ : _____________________________

13. How does the sum of the readings on the scale compare to the combined weight of the bar and the hanger?

14. Move the mass hanger to a different hook and record both readings.

F₁ : _____________________________

F₂ : _____________________________

15. Did the readings on the individual scales change?

16. Did the sum of the readings change?

17. How does the sum of the readings compare to the combined weight of the mass and hanger?

18. Complete the following statement.

The sum of the forces up on the bar ______________ the sum of the forces down on the bar. Thus if an object is in equilibrium, the sum of the forces acting on it is ________.