P8 Forces in Balance

Question 1

What is Newton’s first law?

Answer 1

According to Newton’s first law of motion, an object remains in the same state of motion unless a resultant force acts on it. If the resultant force on an object is zero, this means:

-a stationary object stays stationary
-a moving object continues to move at the same
velocity (at the same speed and in the same direction)

Question 2

What is newton’s second law?

Answer 2

Newton’s second law of motion can be described by this equation:

resultant force = mass × acceleration

This is when:

force (F) is measured in newtons (N)
mass (m) is measured in kilograms (kg)
acceleration (α) is measured in metres per second squared (m/s2)
The equation shows that the acceleration of an object is:

proportional to the resultant force on the object
inversely proportional to the mass of the object
In other words, the acceleration of an object increases if the resultant force on it increases, and decreases if the mass of the object increases.

Question 3

What is inertial mass?

Answer 3

A measure of how difficult it is to change the velocity of an object. It is defined as the ratio of force over acceleration.

Question 4

What is newton’s third law?

Answer 4

Newton’s third law
According to Newton’s third law of motion, whenever two objects interact, they exert equal and opposite forces on each other.

This is often worded as ‘every action has an equal and opposite reaction’.

Question 5

What is a vector quantity and what is a scalar quantity.

Answer 5

Scalar quantities have only magnitude
while Vector quantities have direction as well.

Question 6

What is a moment?

Answer 6

A turning effect of a force.

Question 7

What is a pivot?

Answer 7

A point around which something can rotate or turn.

Question 8

How to calculate the magnitude of a moment?

Answer 8

moment of a force = force × distance

This is when:

moment (M) is measured in newton-metres (Nm)
force (F) is measured in newtons (N)
distance (d) is measured in metres (m)

Question 9

How to increase the size of the moment?

Answer 9

Increase size of force
Increases distance between pivot and effort

Question 10

What is a lever?

Answer 10

A simple machine consisting of a pivot, effort and load.

Question 11

What is an effort?

Answer 11

Force used to move a load over a distance.

Question 12

What is a load?

Answer 12

The overall force that is exerted, usually by a mass or object, on a surface.

Question 13

What is a force multiplier?

Answer 13

Something that increases the effect of a force.

Question 14

What are gears?

Answer 14

A toothed wheel used with other gears to turn axles at different speeds.

Question 15

How do gears work?

Answer 15

As one gear turns, the other gear must also turn. Where the gears meet, the teeth must both move in the same direction. In the diagram, the teeth of both gears move upwards. This means that the gears rotate in opposite directions.

The forces acting on the teeth are identical for both gears, but their moments are different:

If the driven gear is made larger is will rotate more slowly but with a greater moment. For example, a low gear ratio on a bike or car.
If the driven gear is made smaller it will rotate more quickly but with a smaller moment. For example, a high gear ratio on a bike or car.

Question 16

An object in equilibrium will not ________?

Answer 16

turn or accelerate - there is no overall (resultant) force and the clockwise moments are equal to the anticlockwise moments.

Question 17

What is the centre of mass?

Answer 17

The point where its mass is thought of being concentrated.

Question 18

What happens if an object at rest doesn’t turn?

Answer 18

The sum of anticlockwise moments around a point= The sum of clockwise moments around a point

Question 19

What are the conditions for an object to be in equilibrium?

Answer 19

Translational equilibrium: Resultant force = 0.

Rotational equilibrium: Sum of clockwise moments = Sum of anticlockwise moments.
Example: A balanced seesaw has equal moments on both sides.

Question 20

How is a moment calculated, and what determines its direction?

Answer 20

Formula: Moment = Force × Perpendicular distance to pivot.

Direction:

Clockwise: Force tends to rotate object rightward.

Anticlockwise: Force tends to rotate object leftward.

Question 21

How is the resultant force of two vectors found?

Answer 21

Method: Draw vectors as adjacent sides of a parallelogram → Resultant = Diagonal.

Question 22

What does “resolving a force” mean?

Answer 22

Splitting a force into perpendicular components (e.g., horizontal/vertical).

Method: Use a rectangle where diagonal = original force.

Question 23

How to check equilibrium for non-parallel forces?

Answer 23

Resolve forces into two perpendicular directions (e.g., x and y axes).

Sum of components in each direction must = 0.

Question 24

What are component forces?

Answer 24

One of two forces, at right angles to each other, that can be added together to form a resultant force.

Question 25

How does a bottle opener multiply force?

Answer 25

Input force acts farther from pivot → output force on cap is larger.

Question 26

How do gears multiply torque and affect speed?

Answer 26

Torque depends on gear radius: Larger gear = greater torque (but slower speed). Smaller gear = less torque (but faster speed). Key Idea: Gears trade speed for torque (like levers).

Question 27

What is torque?

Answer 27

A rotational force that causes an object to turn around a pivot (axis).

Question 28

Why is cycling uphill harder?

Answer 28

eight Resolution: Gravity’s force splits into two components: Downhill Component (parallel to slope): Must be overcome by pedaling force. Formula: F downhill =mgsin(θ). Normal Component (perpendicular to slope): Presses into the slope. Formula: F normal=mgcos(θ)

Question 29

Tip for memorising resolution of forces in a incline/decline

Answer 29

"Sine = Sliding (downhill), Cosine = Crushing (normal force)."

Question 30

What is a fulcrum?

Answer 30

Definition: The fixed pivot point around which a lever rotates. Role: Determines the mechanical advantage of a lever by changing the relationship between input/output forces.

Question 31

What are "effort" and "load" in moments?

Answer 31

Effort (Input Force): The force you apply to a machine (e.g., pushing down on a lever, pulling a rope). Example: The hand force on a crowbar. Load (Output Force/Resistance): The force the machine overcomes (e.g., weight of an object, friction). Example: The rock being lifted by the crowbar.