Impulsive Forces

Question 1

what is an impulsive force

Answer 1

where forces can be very large as the time during which they act can be very small

Question 2

for a force-time graph, what does the area under the line give you

Answer 2

• the momentum change between the time the line starts at and ends
• or just the impulse force

Question 3

how do you know that the area under the line gives you the impulse force

Answer 3

• the units of the calculated area would be Ns as your multiplying force and seconds
• and the unit for impulse is Ns

Question 4

what would the area of a vertical strip drawn under the line give you

Answer 4

the change in momentum in the time the strip covers in the x axis

Question 5

what would the force-time graph for a all being hit by a tennis racket look like and why

Answer 5

• a hill (like continuous distribution)
• because the force on the ball increases as the strings are stretched
• and then it reduces to 0 as the strings straighten and the ball leaves the racket

Question 6

what can the average size of a force be estimated using

Answer 6

• the impulse-momentum equation

- Ft = mv

Question 7

what two conditions need to be met in order for the impulse-momentum equation to be used to estimate the average size of a force

Answer 7

• the time of contact needs to be known

- the change in momentum needs to be measured

Question 8

other than the equation telling you so, why should you know that Ns = kgms-1 using equations you already know

Answer 8

• the equation for force is F=ma
• in which that calculation has the units kgms-2
• meaning N=kgms-2
• so when you multiple both sides by s you get the units for the impulse-momentum equation
• N(s) = kgms-2(s) which is Ns=kgms-1

Question 9

what is the crumple zone in front of a car there for

Answer 9

• increasing the time it takes for the car to come to a halt
• to reduce the impulse force and therefore the chance of the driver getting injured
• shown in F = (mv - mu) / t
• if you increase time, force decreases

Question 10

a stone is falling freely due to gravity. it is accelerating so its momentum should be continually changing. however this is a closed system, so how does the conservation of momentum apply in even this case

Answer 10

• using newtons third law, the gravitational force applied by the earth on the stone is equal and opposite to the gravitational force applied to the earth by the stone
• this means that a downwards change in momentum of the stone is equal to the upwards change in momentum of the earth
• but because the mass of the earth is huge, there is no noticeable change in the earths velocity or momentum