Forces

Equilibrium

When the Net Force on an Object Equals Zero

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Equilibrium — the state in which the resultant (net) force acting on an object equals zero

Two forces acting on an object

What is the resultant of these 2 forces?

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G. Equilibrium

resultant = 0 N

Balanced judicial scale

Ex 1

Given

What is the resultant of these 2 forces?

Two forces acting on an object
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At Equilibrium:

Fnet = 0, a = 0

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Kids breaking a wishbone

Velocity is Constant (rest or constant v)

Ex 2

Given

Could the forces of 8 N, 6 N and 4 N ever produce a state of equilibrium?

8 N, 6 N and 4 N force diagram

If so, find the resultant of the forces 4 N and 6 N.

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Answer: Yes

Resultant of 4 N and 6 N = 8 N

Ex 3

Given

Could the angles of these forces ever be arranged to produce equilibrium?

4 N and 5 N force diagram at various angles
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Answer: NO

4 N and 5 N have a range of possible resultants: 1 N to 9 N

Robin Hood vs. Little John tug of war

Ex 4

Given

Could these forces ever be in equilibrium?

50 N and 15 N force diagram
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Answer: No

50 N and 15 N have a range of 65 N to 35 N

Carl and Johnny playing king of the mountain

Which Diagram Shows Equilibrium?

Given Question mark character

Which diagram represents a box in equilibrium?

Diagram 1

Diagram 1 - box with forces

Diagram 2

Diagram 2 - box with forces

June 2008 #9

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Answer explanation diagram
Answer

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Check Your Understanding

5 quick questions — pick an answer to see if you’re right.

1. When is an object in equilibrium?

2. A hockey puck slides across frictionless ice at a constant 5 m/s. Is it in equilibrium?

3. Which pair of forces, acting alone on an object, could produce equilibrium?

4. Forces of 8 N, 6 N and 4 N act on an object. Could they produce equilibrium?

5. Forces of 4 N and 5 N act on an object. What third force could put it in equilibrium?

NYSSLS Regents Practice · Forces · Systems in Balance

Hanging a Traffic Light

A traffic light with a weight of 200. N hangs at rest from two identical cables. Each cable makes an angle of 30.° above the horizontal, as shown in Diagram 1.

Diagram 1: Traffic Light Support

30.°30.°

Which statement correctly describes the forces acting on the traffic light?

▶ Want a challenge? Open 6 more Regents-style questions  (harder · mixed formats)

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Multiple choice

C1. Two forces, 30. N east and 40. N north, act on an object. Which single force would put the object in equilibrium?

Determine the value

C2. Each cable pulls on the light with a tension of 200. N at 30.° above the horizontal. Determine the magnitude of the horizontal component of the tension in one cable.

Tx = N
Support or Refute · Justify

C3. A student makes the following claim.

Claim: If the cables were attached higher on the poles so each made 60.° above the horizontal, the tension in each cable would be less than the light’s 200.-N weight.

Place a check mark in either the Support or Refute box. Justify your response using a mathematical relationship.

Substitute values

C4. A different intersection uses the same 200.-N light, but each cable makes 45° above the horizontal. Substitute numerical values into a mathematical representation that can be used to determine the tension in each cable. Then calculate the tension.

T = Fg2 sin θ = N2 sin ° = N
Multiple choice

C5. Three forces act on an object. Which set of force magnitudes could produce equilibrium?

Constructed response

C6. A city engineer must choose between nearly horizontal cables (10.° above horizontal) and steeper cables (30.°) that require taller poles. Identify one benefit and one drawback of choosing the nearly horizontal cables.

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