C4 - Force, Mass and Weight

Question 1

How is centre of mass / gravity calculated?

Answer 1

The plumb line and bob experiment

The centre of mass is a point through which any externally applied force produces straight line motion and no rotation.

Question 2

What are the 6 key forces on free body force diagrams?

Answer 2

Weight (vertically downwards)

Friction (against motion)

Drag (against motion)

Tension (within rope or cable)

Upthrust (vertically upwards)

Normal (perpendicular to ground)

Question 3

How is weight resolved into two parallel and perpendicular components?

Answer 3

Parallel:

• W sin ø
• Fx = mg sin ø

Perpendicular:

• W cos ø
• Fy = mg cos ø

Question 4

What is drag?

Answer 4

A frictional force that opposes the motion of the object through the fluid.
It depends on the speed and area of the moving object.

Question 5

What’s terminal velocity?

Answer 5

When the drag force is equal and opposite to its weight and there’s zero acceleration so speed is constant.

Question 6

What’s a moment?

Answer 6

The turning affect of a force around a point.

The product of force and perpendicular distance from a pivot.

Question 7

What’s a couple?

Answer 7

A pair of equal and opposite forces acting on a body but not in the same line / in opposite directions.
(Anti-parallel forces)

Question 8

What’s torque?

Answer 8

The moment of a couple

Torque (of a couple) is the product of one of the forces of a couple and the perpendicular distance between forces.

Question 9

How is a moment calculated?

Answer 9

Moment = Fx

Question 10

How is force calculated?

Answer 10

F = am

Therefore:
W = mg

Question 11

What is the principle of moments?

Answer 11

The sum of clockwise moments is equal to the sum of anti-clockwise moments when an object is in rotational equilibrium so the net moment is zero.

Question 12

How is the perpendicular distance of a moment calculated?

Answer 12

By using trigonometry

Question 13

How is torque of a couple calculated?

Answer 13

Torque = one of the forces * the perpendicular separation between the forces

= Fd

Question 14

What is density?

How is it calculated?

Answer 14

The defined mass per unit volume of a substance.

p = m / v

Question 15

How is pressure calculated?

Answer 15

P = F / A

Pressure is the normal force exerted per unit cross sectional area

Question 16

How is the pressure of a vertical column of any liquid calculated?

Answer 16

From it’s weight and cross sectional area of its base

P = hpg
Pressure = height of liquid column * density of fluid * acceleration of free fall (9.81)

Question 17

What is upthrust?

Answer 17

The upward buoyant force exerted on a body immersed in a fluid.

Question 18

How is upthrust calculated?

Answer 18

Upthrust = Axpg

= cross sectional area * distance * fluid density * acceleration of free fall

Question 19

What is Archimedes’ principle?

Answer 19

Upthrust exerted on a body immersed in a fluid is equal to the weight of the fluid that the body displaces.

Question 20

Why would the weight of an object not act in the middle of the object?

Answer 20

Distribution of mass is not uniform.

Question 21

If two people are holding an object with non-uniform distribution of mass and person B (further from the centre of mass) moves closer to the centre, what happens to forces on person A and B?

Answer 21

The turning effect of B is less therefore B needs a greater force to produce the same moment (in order to be in equilibrium)

Question 22

What is a newton?

Answer 22

When 1kg mass has an acceleration of 1m/s

Question 23

Describe, in terms of forces acting on the driver how wearing a seat belt and having an air bag in a car can help to protect the driver from injury in a collision:

Answer 23

Impact time increases

Force on the driver decreases

Prevents them colliding with the windscreen

Wider area of airbag (and seatbelt) reduces pressure

The air bag deflates quickly to prevent whiplash

Question 24

What factors affect braking distance?

Answer 24

Condition of road - icy/wet etc

Condition or car/tyres

Mass of car

Speed of car

Question 25

How is GPS used to locate the position of a carL

Answer 25

The distance of the car from different satellites is determined by using electromagnetic waves (radio and micro).

Trilateration used

Delay time from GPS to satellite determines distance/position of car.

Question 26

Why might an object falling from a pulley have an acceleration less than acceleration of free fall / g?

Answer 26

There is tension in the wire which acts as a resistive force / there’s a vertical, opposing.

Question 27

State why the equation F = ma cannot be applied to particles traveling at speeds very close to the speed of light?

Answer 27

The mass of particles increases as speed nears the speed of light

Question 28

How does total force opposing the motion of a car change when it travels up a hill?

Answer 28

The car’s weight opposes motion / more work is done as the car gains GPE

Question 29

What experiment did Galileo carry out which overturned Aristotle’s ideas of motion?

Answer 29

He dropped objects of different masses from the tower of Pisa.

He timed how long it took for them to hit the ground and found they both hit the ground at the same time.

Acceleration is the same for all objects. (Disproving Aristotle’s concept that heavier objects fall faster).