Chapter 4 - Forces in Action

Question 1

Define the Newton?

Answer 1

One newton is defined as the force that will give a mass of 1kg an acceleration of 1 ms^-2
F - ma

Question 2

Define weight?

Answer 2

The weight of an object is defined as the gravitational force acting upon the object.
W = mg
g = 9.81 on earth.

Question 3

What are free body diagrams?

Answer 3

Free body diagrams so all the individual forces acting on an object rather than the resultant force.

Question 4

Define the Normal Contact force?

Answer 4

The normal contact force acts perpendicular to a surface, and is equal and opposite to any force exerted onto the surface.

Question 5

Define tension

Answer 5

Tension is the force from pulling at either end of a material in opposite directions.

Question 6

Define friction

Answer 6

Friction opposites the motion of an object and is the lost of kinetic energy to thermal energy.

Question 7

How is force proportional to acceleration?

Answer 7

Net force(N) = Mass(kg) x Acceleration(ms^-2)
F = ma

Question 8

What is the acceleration on earth?

Answer 8

Earths acceleration is its gravity, 9.81. This means that all object will fall at the same rate (unless acted upon by more resistive forces).

Question 9

What does the triangle of forces represent?

Answer 9

The triangle of forces represents 3 different forces acting on the same object.
The they form a complete triangle (when drawn to scale) the object is in equilibrium.
This can be represented as F1 + F2 + F3 = 0

Question 10

How can you resolve two forces acting on an object by the triangle of forces?

Answer 10

You can find the resultant force by making a triangle, with the head of one force marry up with the tail of the other force. Then resolving to find the missing side.

Question 11

What is the difference between mass and weight?

Answer 11

Weight is vector and mass is scalar.
Weight is measured in newtons.
Mass is measured in kilograms.

Question 12

What is centre of mass?

Answer 12

The centre os mass of an object is the single point that you all the weigh of an objects acts through.

Question 13

How can you experimentally find the centre of mass?

Answer 13

• Hang an object free from a point, using a plumb line draw a line vertically down from this point.
• Repeat this hanging the object from a different point.
• Where the lines overlap is the centre of mass.

Question 14

How is the centre of mass related to the stability of an object?

Answer 14

The higher the centre of mass and the thinner the base area of an object; the more unstable that object is going to be.

Question 15

What are the two types of friction?

Answer 15

Contact friction - Between solid surfaces.
Drag - Depends on the viscosity of the fluid.
Both are:
- Dependant on the surface area of the object.
- Resist motion.
- Never cause something to start moving.
- Convert kinetic energy into thermal energy.

Question 16

What is terminal velocity and when is it reached?

Answer 16

Terminal velocity is when the driving force equals the frictional forces, meaning the object is at its maximum velocity.
The three phases:
Acceleration –> increasing frictional forces –> resistance forces equals driving force.

Question 17

Describe a practical that can be used to measure terminal velocity.

Answer 17

• Get a long thin glass tube, make marks up the tube at fixed distances.
• Put a liquid in the glass tube.
• Drop a ball bearing down the tube, and record the time when the ball passes each mark.
• Work out the velocity between each mark.
• You should find the ball increases in velocity until it comes constant at its terminal velocity.

Question 18

What effects terminal velocity?

Answer 18

• Fluid the object is in - more viscous the slower the terminal velocity will be.
• Shape of the object - higher surface area the slower the terminal velocity.
• Mass of the object - the heavier the object the more frictional force is need to balance the forces. therefore a faster terminal velocity.

Question 19

Define Density.

Answer 19

Density is defined as the mass of an object per unit of volume.
p = m/V
Units can be g/cm^3 or kg/m^3

Question 20

Define pressure.

Answer 20

Pressure is defined as amount of force applied per unit area.
p = F/A
Units Nm⁻² or Pa

Question 21

Define pressure in a fluid?

Answer 21

Pressure in a fluid is defined as the depth of a fluid multiplied by density of the fluid and gravity.
p = hpg

Question 22

What is upthrust and why do we get it?

Answer 22

Upthrust is an upward force that fluid exerts on objects which are submerged in the fluid.
It happens because of the differing pressure values at the top and bottom of the object.
Upthrust or Net force = (h2 - h1)pgA

Question 23

What is archimedes principle?

Answer 23

Archimedes principle states that when a body is submerged in water, it experiences an upthrust force equal to the weight of fluid it has displaced.
This is because:
(h2 - h1)pgA = Vpg = mg = W

Question 24

Define the moment of a force?

Answer 24

The moment of a force is defined as the force multiplied by the perpendicular distance to the pivot to the line of action of the force.
M = F x d

Question 25

What is the Principle of Moment?

Answer 25

The principle of moments states that for a body to be in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about the same point.

Question 26

Define a Couple in terms of moments?

Answer 26

A couple is a pair of forces of equal sizes which act parallel to each other, but in opposite directions.

Question 27

Define Torque of a couple.

Answer 27

Torque is defined as magnitude of one of the forces multiplied by the perpendicular distance between the forces.
T = F x d