Everything Science Grade 11

CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.3

Newton’s Second Law of Motion ESBEY

According to Newton I,things ’like to keep on doing what they are doing’. In other words, if anobject
is moving, it tends to continue moving (in a straight line and at the same speed) and if an object is
stationary, it tends to remain stationary. So how do objects start moving?

Let us look at the example of a 10 kg box on a rough table. If we push lightly on the box as indicated in
the diagram, the box won’t move. Let’s say we applied a force of 100 N, yet the box remains stationary.
At this point a frictionalforce of 100 N is actingon the box, preventingthe box from moving. If we
increase the force, let’ssay to 150 N and the box almost starts to move,the frictional force is 150 N.
To be able to move the box, we need to push hard enough to overcome the friction and then move the
box. If we therefore apply a force of 200 N remembering that a frictional force of 150 N is present,
the ’first’ 150 N will beused to overcome or ’cancel’ the friction and the other 50 N will be used to
move (accelerate) the block. In order to accelerate an object we must have a resultant force acting on
the block.

box

rough table

applied force

Now, what do you thinkwill happen if we pushed harder, lets say 300 N? Or, what do you thinkwill
happen if the mass of the block was more, say 20 kg, or what if it was less? Let us investigate how the
motion of an object is affected by mass and force.

Activity: Newton’s Second Law of Motion

Aim:

To investigate the relation between the acceleration of objects and the application of a constant

resultant force.

Method:

60 ◦

1. A constant force of20 N, acting at an angle of 60◦to the horizontal, is applied to a
   dynamics trolley.


2. Ticker tape attachedto the trolley runs through a ticker timer of frequency 20 Hz as the
   trolley is moving on thefrictionless surface.


3. The above procedureis repeated 4 times, eachtime using the same force, but varying the
   mass of the trolley as follows:
   
   
   • Case 1: 6,25 kg
   
   
   • Case 2: 3,57 kg
   
   
   • Case 3: 2,27 kg
   
   
   • Case 4: 1,67 kg