Chapter 4 Forces In Action

Question 1

How do you calculate the net force of an object?

Answer 1

F=ma

Force = Mass x Acceleration

Question 2

What is the difference between mass and weight?

Answer 2

Mass is constant and a physical property

Weight is the gravitational force acting on an object and is equal mass x gravitational force

Question 3

What is the centre of gravity?

Answer 3

The point at which the resultant gravitational force on an object acts through

Question 4

How do you find the centre of gravity of an object?

Answer 4

Using a plumb-line

When an object is suspended on a piece of string it comes to a rest when the string is vertical

Question 5

What is the centre of mass and how do you find it?

Answer 5

Where all the lines of symmetry intersect

Question 6

How do you work out the forces parallel and perpendicular to a slope on a free body diagram?

Answer 6

Force parallel to the slope = WsinΘ
Force perpendicular to the slope = WcosΘ

Remember W = mg , f=ma and that the normal contact force is equal to the force acting perpendicular to the slope but in the opposite direction

Question 7

What is a drag force?

Answer 7

A frictional force that opposes motion
Its magnitude depends on several things like the speed, shape and texture of the object as well as the density of the fluid it travels through

Speed of object and cross sectional area are two most important

Question 8

How are drag and cross sectional area related?

Answer 8

Large cross sectional area = More drag

Question 9

How are speed and drag related?

Answer 9

Drag os directly proportional to speed^2

Question 10

What happens to an object falling through the air?

Answer 10

Weight remains constant
Speed increases
Drag increases
At the instant the object is dropped there is no drag and the total force is equal to the weight
Acceleration = g
Speed and drag increase till terminal velocity where they are equal and opposite

Question 11

What is the moment of a force?

Answer 11

The turning effect of a force about an axis or point

Question 12

What is the equation for a moment?

Answer 12

Moment = Fx

Moment = Force x Perpendicular distant from the axis

Question 13

When is a body in equilibrium?

Answer 13

No resultant force in any plane

No moment about any point - clockwise and anticlockwise moments about any point are equal

Question 14

What is a couple (forces)?

Answer 14

Two forces that are equal but opposite in direction that can make an object spin

Question 15

What is torque?

Answer 15

The moment of a couple

Torque of a couple = One of the forces x perpendicular separation between the forces

Question 16

How do you resolve a force into vertical and horizontal components?

Answer 16

Vertical = Fcosθ
Horizontal = Fsinθ

Question 17

What is density?

Answer 17

Density is mass per unit volume

P=m/v
Density = Mass/Volume

(Kgm^-3)

Question 18

What is pressure?

Answer 18

The normal force exerted per unit cross sectional area.

P=F/A
Pressure = Force / Area
Nm^-2 or Pa

Question 19

How do you calculate the pressure exerted by a column of liquid?

Answer 19

P = hpg

Pressure = height of liquid column x density of liquid x acceleration of free fall (9.81)

Question 20

How do you get the equation P=hpg?

Answer 20

The pressure at the base of liquid is equal to the weight of the column divided by A

W = Mass of column x g

Mass = density x volume and volume = A x h

So P = p x A x h x g / A and A’s cancel out so

P = hpg

Question 21

What is the equation for upthrust?

Answer 21

Upthrust = Axpg

Volume (Ax) x density x acceleration due to free fall

Question 22

What is archimedes principle?

Answer 22

The upthrust exerted on a body is equal to the weight of the fluid displaced by it

Question 23

When does an object sink?

Answer 23

If the upthrust is less than the weight of the object

If the object floats then the weight is equal to the weight of fluid it displaces