Forces In Action 3.2

Question 1

Define the centre of mass of an object.

Answer 1

An imaginary point of an object where the entire mass of the object appears to act.
Or
An imaginary point of an object through which any externally applied force produces a straight line motion but no rotation.

Question 2

The centre of gravity.

Answer 2

The centre of gravity coincides with the centre of mass on Earth.

Question 3

What are the factors that affect drag of an object in a fluid?

Answer 3

Speed of the object, cross sectional area of the object, the object’s shape and texture, and the density of the fluid

Question 4

What is meant by when an object reaches terminal velocity?

Answer 4

When the drag force of the object is equal and opposite to its weight, meaning that acceleration is 0 m/s/s and, thus, at constant speed.

Question 5

Describe the motion of a free falling object.

Answer 5

During a vertical fall through air or another fluid, the weight of the object remains constant, but the drag force increases as the speed increases.

At the instance an object starts to fall there is no drag force on the object. The total force is equal to the weight and the acceleration of the object is g, the acceleration of free fall.

As the objects falls, the speed increases, in turn increasing the magnitude of the opposing drag force. The resultant net force of the object decreases and the instantaneous acceleration of the object becomes less than g. (a= (mg - D) /m and F = mg - D).

Eventually, the object will reach terminal velocity, when the drag force on the object is equal to its weight. At terminal velocity, the object has 0 acceleration and constant speed.

Question 6

What is the moment of a force ?

Answer 6

The moment of a force is the turning effect of a force about some axis or point.

Question 7

What are the conditions that establish if a body is in equilibrium or not?

Answer 7

The net force acting on the body is 0 and the net moment is also 0.

Question 8

Define the principle of moments.

Answer 8

For a body in rotational equilibrium the sum of the clockwise moments about any point is equal to the sum of the anticlockwise moments about the same point.

Question 9

Define a couple.

Answer 9

Two equal and opposite parallel forces that act on an object through different lines of action. It has the effect of causing rotation with no translation

Question 10

Define a torque.

Answer 10

A torque is the moment of a couple and it is calculated by multiplying one of the forces by the perpendicular distance between them.

Question 11

Define pressure.

Answer 11

Pressure is the normal force exerted per unit cross sectional area

Question 12

What is the second formula for pressure of a fluid ?

Answer 12

p=hpg

Question 13

Derive the formula p=hpg

Answer 13

p=F/A
F=W=mg
m=pV
V=Ah
F=pAhg
p=pAhg/A=hpg

Question 14

Define archimedes principle.

Answer 14

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

Question 15

The formula to calculate the upthrust of an object submerged partially or fully in a fluid ?

Answer 15

upthrust = Axpg

Question 16

Derive the formula upthrust=Axpg from the formula p=hpg

Answer 16

The pressure in the upper surface is hpg, thus,
The force experienced is hpgA,

The pressure in the lower surface is (h+x)pg, thus,
The force experienced is (h+x)pgA.

The difference between forces = weight displaced = upthrust for bodies in equilibrium.

upthrust=(h+x)pgA - hpgA = Axpg

Question 17

the final speed v of an object is

Answer 17

v=√2gh

Question 18

Drag is proportional to…

Answer 18

the square of the speed, and it also increases as the cross-sectional area increases.

Question 19

Describe an experiment to determine terminal velocity of a ball in viscous fluid.

Answer 19

The terminal velocity of a ball bearing in viscous fluid can be determined experimentally.

Take a cylinder and fill it with a viscous liquid.
Hold a ball bearing over the surface of the liquid, and then release it from rest.
At the same time, start a timer, and mark the position of the ball bearing within the tube.
When the ball bearing reaches terminal velocity, the distance it has travelled between each time interval will be the same, and so its velocity can be determined.

Question 20

What can you do to increase the accuracy of an experiment to determine terminal velocity of a ball in viscous fluid?

Answer 20

To increase the accuracy of this experiment, set up a pulley system over the tube of fluid, with the ball bearing attached on one end.
On the other end, attach a strip of tape passing through a ticker timer.
The ticker timer will produce 50 dots per second on the tape, which will travel at the same speed as the ball bearing.
The distance between dots and the time can then be used to determine terminal velocity.

Question 21

Friction

Answer 21

the force that arises when two surfaces rub against each other.

Question 22

Weight

Answer 22

the gravitational force acting on an object, through its center of mass.

Question 23

Drag

Answer 23

the resistive force on an object travelling through a fluid (e.g. water or air).

Question 24

Tension

Answer 24

the force within a stretched cable or rope.

Question 25

Up-thrust

Answer 25

the upward buoyancy force acting on an object when it is in a fluid.

Question 26

Normal Contact Force

Answer 26

the force arising when an object rests against another object
acting at a 90° angle to the plane of contact.

Question 27

Two people apply forces A and B at each end of the lawn mower. The weight of the lawn mower is 350 N. Explain why the weight of the lawn mower does not act in the middle of the
lawn mower.

Answer 27

The (distribution of the) mass of the lawn mower is not uniform

Question 28

Define the newton.

Answer 28

(Force is 1 N) when a 1 kg mass has an acceleration of 1 m s–2

Question 29

Describe in terms of the forces acting on the driver how wearing a seat belt and having an airbag in a car can help to protect the driver from injury in a head on collision.

Answer 29

Four from:
Prevents the driver from hitting the steering wheel / windscreen
Deflates quickly to prevent whiplash
Increases the time/distance to stop
Decreases the (impact) force on the driver
Much wider area of the bag reduces the pressure

Question 30

Describe how Global Positioning System (GPS) is used to locate the position of a car on the Earth’s surface.

Answer 30

1. (Several) satellites used
2. Distance from (each) satellite is determined
3. Position / distance is determined using c / speed
   of e.m waves / radio waves / microwaves and
   delay time (wtte)
4. Trilateration is used to locate the position of the
   car
   Or position of car is where circles / spheres cross
   (wtte)

Question 31

State why the equation ‘F = ma’ cannot be applied to particles travelling at speeds very close to the speed of light.

Answer 31

The mass of particles increases (at its speed gets closer to the speed of light)

Question 32

Describe an experiment to determine the center of gravity of the metal plate shown in the figure below.

Answer 32

Any three from:
1. (Suspend plate from a point and then) mark a vertical line on the plate
2. Plumb line / ‘pendulum’ (used to find the vertical line)
3. Hang from another point / place (and draw another vertical line)
4. Where the lines intersect gives position of center
of gravity