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2.6 Impulsive Force

Definition and S.I. unit  for Impulse

 

Impulse  is defined  as the change in momentum

 

Impulse     =  m ( v – u )

                                   m = mass

                                   v  = final velocity

                                   u  = initial velocity

 

Unit of  impulse = kg ms -1  or  N s

Impulse is a vector quantity.

 

From Newton’s second law of motion,

F =  m ( v – u )

                t

Ft = m( v – u ) = change in momentum

 

Impulse is also defined as the product between impulsive force,F  with the time impact, t.

or

Impulse = Ft

                         F = Impulsive force

                          t = time impact

clip_image002

Impulse = area under graph F – t

Example 1

 

 

An object of mass 5 kg falls from a building. Its velocity is

6 ms-1 before it touches the ground. How much is the impulse on the object once it hits the ground.

Solution

 

 

 

 

Example 2

 

Figure shows a trolley of mass 1.5 kg hits the wall with a velocity  5 ms-1 and bounches back with a velocity of 4 ms-1.

    clip_image004

Calculate the magnitude of the impulse during collision.

 

Solution

 

 

 

Example 3

 

A box of mass 2 kg is pulled with a force of 40 N for  5 s. How much is the impulse upon the object.

 

Solution

 

 

Definition and S.I. unit  for Impulsive Force

 

 

The impulsive force  is a large force which acts over a very short time interval.

Impulsive force is defined as the rate of change of momentum .

 

Impulsive force , F = Change of momemtum

                                             Time

                                                  F = m ( v – u )

                                                              t

                                                        m = mass

                                                        v  = final velocity

                                                        u  = initial velocity

                                                        t   = time impact

Unit of  impulsive force  = kg ms2   or N

Impulse force  is a vector quantity.

 

Impulsive force also defined as the rate of impulse

 

Impulsive force , F =  Impulse

                                             Time

clip_image006

   Impulsive force = gradient of the graph Impulse – Time

Example 4

 

 

A tennis ball of mass 100 g  is moving at a velocity of 40  ms1.

A player hits the ball and moves in the opposite direction with a velocity of 10 ms 1. How much  is the impulsive force experienced by the ball if the time of collision is 20 m s.

 

Solution

 

 

 

 

 

Example 5

 

 

A boy of mass 50 kg falls a high place to the ground. The velocity of the boy when he just touches the ground is  4  ms-1.

What is the impulsive force acting on the boy if

(a)     he bends his knees  upon landing on the  ground and takes 0.5 s to stop

(b)     he lands  on the ground with stiff legs and takes 0.02 s to stop

 

Solution

 

 

Effect of time interval of impact on size of impulsive force

 

 

From the formula for implusive force,

 

                           F = m ( v – u )

                                           t

we can see that when the stopping time ,t , is small, F is big ; and when t  is big , F is small.

The impulsive force is inversely proportional to the time of impact.

 

Reducing impulsive forces

 

1.      A gymnast bends her knees as he lands, the momentum change occurs over a longer period of time, and so the impulsive force will be lower and to avoid breaks her leg.

 

2.      In games like baseball, cricket and softball, the catcher always pulls his hand backward when catching the ball to lengthen the collision time and hence reduces the impact of the impulsive force.

 

3.      Thick mattresses with a soft surface are used in events such as the high jump so that the time interval of impact on landing is increased, thus reducing the impulsive force.

 

4.      Items that are fragile, such as eggs, glass and electrical appliances must be packed in materials that are stiff but compressible. These materials are able to absorb and reduce impulsive forces by extending the time interval of the change in momentum during an impact.

 

When impulsive forces are beneficial

 

 

1.      Hammer and nail are made of metal. When a hammer hits at  a nail , the collision time is small and hence the impulsive force is great. So the nail penetrates a wooden material easily.

 

2.      In the field of construction, a heavy metal pile falls on an iron post. When the metal pile hits the iron post , the high rate of change of momentum will cause the big impulsive force to force the iron post into the ground.

 

3.      A pounder and a mortar made from hard stone are used to crush the foodstuffs such as chilli and pepper into small pieces by a big impulsive force.

 

4.      An exponent of karate has the capability to split a thick wooden sled by bringing his hand down hard onto the surface of the wood. The momentary contact produces a great impulsive force which splits the wooden slab.

 

 

2 Responses

  1. assalamualaikum…
    Boleh tak sy mohon jasa baik tuan/puan utk dptkn jawapan bagi soalan fizik?

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