3.2 - Forces in action

Question 1

Describe what happens when a resultant force (F) acts on a body with mass m

Answer 1

The body will accelerate in the direction of the resultant force. The resultant force is related to mass and acceleration by the formula:
F=ma

Question 2

Give 3 examples of common forces and explain briefly what they do

Answer 2

Weight - the gravitational force acting on an object through its centre of mass
Friction - the force that arises when two surfaces rub against each other
Drag - the resistive force on an object travelling through a fluid
Tension - the force within a stretched cable or rope
Upthrust - the upward buoyancy force acting on an object when it is in a fluid
Normal contact force - the force arising when an object rests against another object. It acts perpendicular to the plane of contact

Question 3

What is meant by drag?

Answer 3

An object moving through a fluid experiences a drag force acting on it. Drag is a frictional force that opposes motion. In air this is air resitance

Question 4

What is meant by terminal velocity?

Answer 4

When the forces acting on the falling object (drag and weight) become balanced so the resultant force is 0. The acceleration becomes 0 and the object is moving at maximum velocity

Question 5

What is the principle of moments?

Answer 5

For an object in equilibrium, the sum of clockwise moments is equal to the sum of anticlockwise moments

Question 6

What is a moment

Answer 6

A turning force:

Force x perpendicular distance

Question 7

What is meant by a couple

Answer 7

A pair of equal and opposite coplanar forces that have equal magnitude and opposite direction

Question 8

If you have a uniform object where would its centre of mass be

Answer 8

At the geometric centre of the object

Question 9

Describe what is meant by density

Answer 9

The mass per unit volume of a substance

kg m^-3

Question 10

Describe what is meant by pressure

Answer 10

The pressure is the normal force exerted on a surface per unit area.
Measured in Pascals

Question 11

What is the force that an object submerged in fluid experiences?

Answer 11

It experiences an upwards force called upthrust. This is because the pressure at the bottom is greater than at its top surface

Question 12

State Archimedes principle

Answer 12

The upthrust exerted on a body immersed in fluid whether partially or fully submerged, is equal the weight of the fluid that the body displaces. An object will sink if the upthrust is smaller than the weight. For a floating object its weight is equal to the upthrust