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 tyne 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

3 from:

• 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 (e.g. water or air)
• Tension - The force within a stretched cable or rope
• Upthrust - The upward buoyancy acting in 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 resistance

Question 4

What is meant by terminal velocity?

Answer 4

When the forces acting on the falling object (e.g. drag and weight) become balances so the resultant force is 0. The acceleration becomes zero 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 the clockwise moments is equal to the sum of the anti-clockwise moments

Question 6

What is a moment?

Answer 6

A turning force:

force x perpendicular distance

(perpendicular distance from the point to the line of action of the force)

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, applied to a body parallel to each other but not along the same line

Question 8

If you have a uniform object, where would it’s 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. The unit is kgm^-3

Question 10

Describe what is meant by pressure:

Answer 10

The pressure is the normal force exerted on a surface per unit area. It is measured in Pascals (Pa), where 1 Pa = 1 Nm^-2

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 of the pressure at the bottom surface of the object is greater than at its top surface

Question 12

State Archimedes’ principle:

Answer 12

The upthrust exerted on a body immersed in a fluid, whether partially or fully submerged, is equal to 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, it’s weight is equal to the upthrust