C4 Forces In Action

Question 1

Device used to measure weight

Answer 1

Newton meter, e.g. 1.0kg object = 9.81 newtons.

Question 2

Define the centre of mass of an object

Answer 2

The point through which any externally applied force produces straight-line motion but no rotation.

Question 3

Where does weight act on an object?

Answer 3

It acts on an object’s centre of gravity (which on earth is the same as its centre of mass)

Question 4

How to find an object’s centre of gravity

Answer 4

• Freely hang object from a point along its edge. The centre of mass is vertically below point of suspension.
• Attatch a plumb-line to the same point suspension point.
• Trace a line on the object from the plumb line.
• Suspend on multiple points and the intersection of lines is centre of gravity.

Question 5

Define drag force

Answer 5

The resistive force that opposes the motion of an object travelling through a fluid (e.g. air, water)

Question 6

Define upthrust, and state what determines its magnitude

Answer 6

The upward force extered by a fluid on an object submerged in its, equal to the weight of the fluid displaced by the object.

Question 7

Define normal contact force

Answer 7

A perpendicular force exerted by a surface when an object rests on it.

Question 8

Force parallel to the slope

Answer 8

mg sin θ

Question 9

Force perpendicular to the slope

Answer 9

mg cos θ

Question 10

List factors that determine the magnitude of drag (and which are the most important)

Answer 10

Most Important:
- Speed of object
- Shape of object

• Cross-sectional area of object
• Roughness/texture of object
• Density of fluid

Question 11

Relationship between drag force and speed.

Answer 11

• Drag force is directly proportional to speed^2.
• If speed x2, the drag forces x2^2, so x4.

Question 12

Define Terminal Velocity (and its speed/acceleration)

Answer 12

When the drag force on an object is equal in magnitude and opposite in direction to its weight, therefore speed is constant and acceleration is zero.

Question 13

State the principle of moments

Answer 13

For an object to be in rotational equilibrium, the sum of all clockwise moments about a pivot must be equal to the sum of all counterclockwise moments about that same pivot.

Question 14

What is a couple of forces

Answer 14

A pair of forces equal in magnitude but opposite in direction, applied in parallel but along different lines.

Question 15

Define torque

Answer 15

The moment of a couple, defined as Fd, where F is one of the forces, and d = perpendicular distance separation between the forces.

Question 16

Formula to calculate upthrust

Answer 16

• ρvg, volume of fluid displaced (Archimedes principle).
• density of fluid x volume of displaced fluid x g

Question 17

State Archimedes Principle

Answer 17

• The upthrust exerted on an object immersed in a fluid, whether partially or fully submerged, is equal to the weight of the fluid the object displaces.