0.2 Force, momentum and impulse  (Page 30/35)

Egg throw

This activity demonstrates the effect of impulse and how it is used to improve safety. Have two learners hold up a bed sheet or large piece of fabric. Then toss an egg at the sheet. The egg should not break, because the collision between the egg and the bed sheet lasts over an extended period of time since the bed sheet has some give in it. By increasing the time of the collision, the force of the impact is minimized. Take care to aim at the sheet, because if you miss the sheet, you will definitely break the egg and have to clean up the mess!

Exercise

1. A canon, mass 500 kg, fires a shell, mass 1 kg, horizontally to the right at 500 m $·$ s ${}^{-1}$ . What is the magnitude and direction of the initial recoil velocity of the canon?
2. A trolley of mass 1 kg is moving with a speed of 3 m $·$ s ${}^{-1}$ . A block of wood, mass 0,5 kg, is dropped vertically into the trolley. Immediately after the collision, the speed of the trolley and block is 2 m $·$ s ${}^{-1}$ . By way of calculation, show whether momentum is conserved in the collision.
3. A 7200 kg empty railway truck is stationary. A fertilizer firm loads 10800 kg fertilizer into the truck. A second, identical, empty truck is moving at 10 m $·$ s ${}^{-1}$ when it collides with the loaded truck.
   1. If the empty truck stops completely immediately after the collision, use a conservation law to calculate the velocity of the loaded truck immediately after the collision.
   2. Calculate the distance that the loaded truck moves after collision, if a constant frictional force of 24 kN acts on the truck.
4. A child drops a squash ball of mass 0,05 kg. The ball strikes the ground with a velocity of 4 m $·$ s ${}^{-1}$ and rebounds with a velocity of 3 m $·$ s ${}^{-1}$ . Does the law of conservation of momentum apply to this situation? Explain.
5. A bullet of mass 50 g travelling horizontally at 600 m $·$ s ${}^{-1}$ strikes a stationary wooden block of mass 2 kg resting on a smooth horizontal surface. The bullet gets stuck in the block.
   1. Name and state the principle which can be applied to find the speed of the block-and-bullet system after the bullet entered the block.
   2. Calculate the speed of the bullet-and-block system immediately after impact.
   3. If the time of impact was 5 x 10 ${}^{-4}$ seconds, calculate the force that the bullet exerts on the block during impact.

Torque and levers

Torque

This chapter has dealt with forces and how they lead to motion in a straight line. In this section, we examine how forces lead to rotational motion.

When an object is fixed or supported at one point and a force acts on it a distance away from the support, it tends to make the object turn. The moment of force or torque (symbol, $\tau$ read tau ) is defined as the product of the distance from the support or pivot ( $r$ ) and the component of force perpendicular to the object, $F_{\perp }$ .

Torque can be seen as a rotational force. The unit of torque is N $·$ m and torque is a vector quantity. Some examples of where torque arises are shown in Figures  [link] , [link] and [link] .