Chapter 4, Forces

Question 1

Defining equation for force?

Answer 1

F=ma (force=mass*acceleration).

Question 2

Unit for force?

Answer 2

Newtons.

Question 3

Difference between mass and weight?

Answer 3

Mass (measured in kg) is how much stuff there is, weight (measured in N) is the product of mass and g and gives the force applied to an object towards the centre of mass of the object it is in the gravitational field of.

Question 4

What is the centre of mass?

Answer 4

The point in an object through which weight acts, also if a force is applied through the centre of mass there is no net moment.

Question 5

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

Answer 5

if you hang an object from a point you know the line through the point which is perpendicular to the earth goes through the centre of mass. You can use a plumb-line to help measure this position.

Question 6

What is weight?

Answer 6

The gravitational force acting through the centre of mass of an object.

Question 7

What is friction?

Answer 7

The resistive force from two rough objects rubbing against each other.

Question 8

What is drag?

Answer 8

The resistive force acting on an object travelling through a fluid.

Question 9

What is tension?

Answer 9

The force within a stretched object (usually string/rope)

Question 10

What is upthrust?

Answer 10

An upwards force acting on an object inside a fluid caused by the displacement of this fluid.

Question 11

What is the normal contact force?

Answer 11

The force acting 90 degrees to the surface an object is resting on stopping it falling through the object.

Question 12

On a smooth slope with an object resting on it what is the value of the normal contact force and the component of the weight down the slope?

Answer 12

Normal contact = component of weight perpendicular to slope = mgcos(theta), component of weight down slope = mgsin(theta).

Question 13

What are the main factors that is drag proportional to?

Answer 13

Speed of the object and cross sectional area, also shape of object, roughness/texture of object and the density and viscosity of the fluid.

Question 14

What is terminal velocity?

Answer 14

The point of equilibrium between weight and the drag force such that the net force is 0.

Question 15

What is the equation for the moment of a force?

Answer 15

moment = force * perpendicular distance.

Question 16

Unit for moments?

Answer 16

Newton meters.

Question 17

What is the principle of moments?

Answer 17

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

Question 18

What is a couple?

Answer 18

Two equal forces acting in opposite directions about a point such that it rotates without having any displacement.

Question 19

What is the defining equation for density?

Answer 19

ρ = m/V (density = mass/volume).

Question 20

What is the defining equation for pressure?

Answer 20

p = F/A (pressure = force/area).

Question 21

What is the unit for pressure?

Answer 21

The Pascal (Pa).

Question 22

What does the equation p=hρg show?

Answer 22

pressure = height fluid column * density of liquid * acceleration due to gravity.

Question 23

What is the equation for upthrust?

Answer 23

upthrust = Axρg where A is the area on top, x is the height of the submerged object, ρ is pressure and g is acceleration due to gravity.

Question 24

What is Archimedes principle?

Answer 24

The upthrust exerted on a body in a fluid is equal to the weight of the fluid that body displaces.