P8 - Forces in Balance

Question 1

What is a Vector Quantity?

Answer 1

A vector quantity has a magnitude and a direction.

E.g Acceleration, Force

Question 2

What is a Scalar Quantity?

Answer 2

A scalar quantity only has a magnitude.

E.g Distance, Energy and Speed

Question 3

What is Displacement? How does it differ to distance?

Answer 3

Displacement refers to an object’s overall change in position (it’s a vector).

Distance refers to how much ground an object has covered (it’s a scalar - just magnitude).

Question 4

What are the main forces?

Answer 4

• Weight (Down)
• Upthrust / Normal Force (Up)
• Thrust
• Drag
• Water Resistance/Air Resistance
• Magnetism (Push or Pull)
• Friction

Question 5

What is the resultant force acting an object?

Answer 5

The sum effect of more than one force acting on an object.

E.g ‘X’ is being pushed 50N to the left and 30 N to the right.

The resultant force of ‘X’ is 20N Left.

Question 6

❌ What is a moment?

Answer 6

A moment is a turning effect.

Moment = Force x Distance (from pivot)

M = F x D

(Nm) = (N) x (m)

Question 7

❌ What does the law of moments say? How do you calculate/use the weight and moment of a beam?

Answer 7

If the sum of clockwise moments = sum of anti-clockwise moments, the object is said to be in EQUILIBRIUM

Therefore, on a see-saw:

Weight x Distance of person 1 = Weight x Distance of person 2

W1xD1 = W2xD2

Moment of beam = W0 x D0

W0, weight of the beam
D0, distance from the pivot

W0 x D0 = W1 x D1

W0 = W1 x D1 / D0

Question 8

❌ What is a Lever, Load, Pivot and Effort?

Answer 8

A lever is a simple machine that acts as a force multiplier.

Load = weight of object
Effort = force applied
Pivot = the fixed point at which a lever rotates

Question 9

❌ What is a Gear?

Answer 9

Gears are wheels with rotating teeth that interlock with other wheels. They are used to transmit and often multiply turning forces (moments).

Question 10

❌ What is the equation used to find the moment (turning effect) of a gear?

Answer 10

Moment = Force x Gear wheel Radius

Question 11

❌ How do you apply ratio to gears?

Answer 11

If you know the ratio of the number of teeth for 2 gears, you know the ratio of the diameter, turning effect, and speed

Gear A = 24 teeth  Gear B = 12 teeth
Ratio of Teeth = 2:1
Ratio of  Diameter = 2:1
Ratio of Turning effect = 2:1
Ratio of speed = 1:2

Less teeth = Lower gear = lower turning effect but greater speed.

Question 12

What is meant by the term centre of mass?

Answer 12

The centre of mass of an object refers towhere the mass is concentrated.

Question 13

How do you find the centre of mass of a regular object?

Answer 13

Draw a horizontal line through the middle and a vertical line through the middle and see where they meet.

Question 14

How do you find the centre of mass of an irregular object?

Answer 14

1. Put a hole in one corner of the shape and suspend from a stand rod.
2. Use a plumb line to draw a vertical line down the object.
3. Repeat the procedure by hanging from different corners. (Need at least 3 lines).
4. The point where the lines intersect is the centre of mass.

Question 15

When is an object stable?

Answer 15

When its centre of mass is directly above the base of support.

Question 16

When is an object unstable?

Answer 16

When its centre of mass is not directly above the base of support.

Question 17

What is the parallelogram method used for? How do you use it?

Answer 17

The parallelogram method is used to calculate the resultant force acting on an object, when two forces are not acting along the same line.

How to use it:

1) Complete the parallelogram by drawing 2 parallel lines that are the same length as the lines already drawn. USE CORRECT SCALE.
2) Join the opposite ends of the parallogram with a solid line.
3) Measure the distance between the 2 points.
4) Draw a normal with a dotted line.
4) Measure the angle to the normal between the 2 points.

Write Answer:

• 90N 0° to the horizontal
• 54N 3° to the horizontal

Question 18

How do you resolve a diagonal force into horizontal and vertical components?

Answer 18

You need to are make a right-angled triangle:
-the diagonal force and its angle will be given.

1) Draw the diagonal force using an appropriate scale.
2) Draw dotted lines to both axis from the head of the diagonal line.
3) Draw arrows on the dotted line - must be in the correct direction.
4) Measure the dotted lines and calculate their values using the original scale.

Question 19

How do you resolve the forces acting on an object on a slope?

Answer 19

You have to work out:

• how much force is going into the ground (weight perpendicular)
• how much force is forcing the object up/down the hill (weight parallel).
• the weight of the object on the slope is given

How to do it:

1) Create a scale for the weight given.
2) Draw the weight line to scale.
3) Draw the line for the perpendicular component - can be any length.
4) Now draw a line parallel to the slope for the parallel component - line should be from the end of the weight line to somewhere on the perp perpendicular line.
5) Mark where the perpendicular and parallel lines intersect.
6) Calculate the magnitude of the weight perpendicular and weight parallel using the original scale.