Chapter 4 - couples, torque, density etc.

Question 1

What happens when a resultant force acts on an object

Answer 1

There is an acceleration

Question 2

Equation for force and units for force

Answer 2

F = ma

Newton’s (kgms^-2)

Question 3

Difference between mass and weight and equation for weight

Answer 3

Mass is the amount of stuff- it is constant for a given object (kg)

Weight is the force exerted by an object and is measured in Newtons

W = mg

Question 4

What is a centre of mass

Answer 4

The centre of mass of an object is the point through which any externally applied force produces straight line motion but no rotation

Question 5

Finding centre of mass

Answer 5

• a freely suspended object will come to rest with its centre of mass vertically below the suspension point
• this can be found by hanging a plumb line from the suspension point and drawing the line that it hangs along
• repeat for different points of suspension and the intersection of the lines is the centre of mass

Question 6

Free-body diagrams

Answer 6

Can be used to model all of the forces acting on an object where forces are shown as vector arrows

Question 7

Define Weight

Answer 7

The gravitational force acting on an object, through its centre of mass

Question 8

Define friction

Answer 8

The force that arises when two surfaces rub together forming a resistive force opposite to motion

Question 9

define drag

Answer 9

The resistive force acting on an object travelling through a fluid

Question 10

Define Tension

Answer 10

The force within a stretched cable or rope

Question 11

Define up-thrust

Answer 11

The upward buoyancy of acting on an object in a fluid equal to the weight of fluid displaced

Question 12

Define normal contact force

Answer 12

The force acting when an object rests against another object acting at a 90 degree angle to the plane of contact

Question 13

Main factors affecting size of drag

Answer 13

The speed of the object
Drag directly proportional to speed squared

The cross-sectional area of the object

Question 14

What is terminal velocity

Answer 14

The point in an objects free fall where it’s weight is equal to drag causing there to be 0 acceleration

Drag = mg

Question 15

Techniques to determine terminal velocity

How to improve it

Answer 15

• fill a cylinder with a viscous liquid
• release a ball bearing and start a timer simultaneously
• mark the position of the ball at regular time intervals and when the distance travelled between lines is equal it will have reached terminal velocity

Attach the ball to a pulley system with a ticker tape on the other side

Question 16

Define a moment of a force

Give the equation for the moment of a force

Why might it be necessary to resolve forces

Answer 16

A moment is the turning effect of a force and is defined by

Moment = Fx
Where moment is in Nm
F = force (N)
x = perpendicular distance from pivot

It might be necessary to resolve forces if they aren’t acting perpendicular to the pivot

Question 17

What is the principle of moments

Answer 17

For a body in rotational equilibrium the sum of the clockwise moments is equal to the sum of the anti-clockwise moments

Question 18

What is a couple

Answer 18

A couple is a pair of forces that have equal magnitude and are parallel but act in opposite directions

i.e. they only produce a rotational force and no translational force

Question 19

What is the torque of a couple

Answer 19

The torque of a couple is defined as the product of the magnitude of one of the forces multiplied by the perpendicular distance between them

Question 20

What is the result if 3 forces form a triangle where the arrows are end to end

Answer 20

The resultant force is 0

Question 21

Define density and the equation

Answer 21

Density is the mass per unit volume of a substance

P = m/v

Question 22

Methods of determining density

Answer 22

For regular objects it can be measured with a ruler and weighed

For irregular objects an Archimedes can could be used or displacement where the volume of the object is equal to the water displaced

Question 23

Define pressure and the equation for pressure

Answer 23

Pressure is the normal force exerted on a surface per unit cross sectional area

P = f/a

Question 24

What is atmospheric pressure

Answer 24

About 100,000 pa

Question 25

What is pressure of a vertical column of fluid

Answer 25

P = hpg

Question 26

What is Archimedes principle

Answer 26

The upthrust exerted on a body immersed in fluid is equal to the weight of the fluid displaced

Question 27

define centre of gravity

Answer 27

The point through which the weight (appears to) act

Question 28

why does an object experience upthrust

Answer 28

The object experiences a greater pressure at its bottom surface compared
with its upper surface.
This difference in pressure produces an upward force (upthrust).
The upthrust is equal to the weight of the water displaced – Archimedes’
principle.

Question 29

what to remember when working with F = MA and gravity

Answer 29

if the force acts upwards, remember to take away the weight of the object first

Question 30

what is the component of weight of an object on a slope

Answer 30

still the vertical component, given by wsin(theta), theta is the angle between the slope and the horizontal

Question 31

what is a good way to define the resultant force acting on an object

Answer 31

the force obtained when ‘the vector sum’ of all the forces acting on the object is taken

Question 32

why can seat belts and airbags help prevent injuries in crashes

Answer 32

• increases time period for which change in momentum occurs, therefore decreases force
• increases surface area (airbag), therefore decreases pressure
• stops whiplash
• prevents driver hitting windshield

Question 33

why can we not use F = ma for objects travelling close to the speed of light

Answer 33

the mass of objects travelling close to the speed of light INCREASES