TRY THIS!1. Find the value of F.

Ans: 45N

2. Calculate the value of m.

Ans: 25 kg

3. A 10 kg object accelerates from rest to 5 ms-1 in 2 s. What is the net force acting on the object?

Ans: 25 N

2.6 Analysing impulse and impulsive force

IMPULSIVE FORCE,F

1)An impulsive force is a large force that acts over a short period of time during collision or an explosion.

2) From the formula ,

3) Impulsive force is directly proportional to the change

in momentum inversely proportional to time.

1) Impulse is the product of an impulsive force, F acting on an object of mass, m, and the time of impact, t.

IMPULSE

2) From the equation,

Impulse, Ft, is equal to the change in momentum.3) The SI unit of impulse is newton seconds (N s)

1) From the formula,

2) Example :a) A glass that is dropped and lands on a cement floor will break. As the surface of the floor is hard, the time of impact is very short. Therefore, impulsive force that acts on the glass is very large.

Effect of Time on the Impulsive Force

b) If an identical glass lands on a soft rubber mat, it might not break. The soft surface of the rubber mat increase the time of impact, thus reducing impulsive force.

Situation for Reducing Impulsive force

When the athlete falls on a mattress, a the thick and soft mattress lengthen the time of impact. Thus, impulsive force acting on the athlete is reduced. He is less likely to suffer any injuries.

The parachutist bends his legs to lengthen the time of impact of his legs with the ground

During impact, the soft polystyrene foam will lengthen the time of impact. This will reduce the impulsive force acting on the fragile goods. Thus preventing from breaking.

Situation for Increasing Impulsive force

The pile hammer will be raised to a certain height and then released onto the piles. The high velocity of pile hammer and the collision between the pile hammer and the piles happens in a very short time.Thus, producing large impulsive force.

Exercise

1) A hockey player uses a hockey stick to hit a hockey ball, causing the ball to travel with a velocity of 30 ms-1. If the mass of the hockey ball is 100g and the time of impact between the hockey stick and the ball is 5 x 10-3 s, calculate the impulsive force exerted by the hockey ball.

2) A trolley with a mass of 500 g is at rest on a smooth surface. The trolley is then given a horizontal impulses of 5 N s. What is the velocity of the trolley after the impact?

3) A boy with a mass of 60 kg jumps over a fence. His velocity just before landing on the ground is 10 ms-1. What is the impulsive force on the boy’s legs if he

a) takes 0.5 s to stopb) bends his legs and stops in 2.5 s