2 Mechanics - Pearson Global Schools

Weight: The Earth is pulling the box down with a force W. According to Newton’sthird law, the box must be pulling the Earth up with a force of magnitude W asshown in Figure 2.55.3. Recoil of a gunWhen a gun is fired the velocity of the bullet changes. Newton’s first law impliesthat there must be an unbalanced force on the bullet; this force must come fromthe gun. Newton’s third law says that if the gun exerts a force on the bullet thebullet must exert an equal and opposite force on the gun. This is the force thatmakes the gun recoil or ‘kick back’.4. The water cannonWhen water is sprayed at a wall from a hosepipe it hits the wall and stops.Newton’s first law says that if the velocity of the water changes, there must be anunbalanced force on the water. This force comes from the wall. Newton’s thirdlaw says that if the wall exerts a force on the water then the water will exert a forceon the wall. This is the force that makes a water cannon so effective at dispersingdemonstrators.NWFigure 2.55 Forces acting on the Earthaccording to Newton’s third law.A boat tests its water cannons.Exercise32 Use Newton’s first and third laws to explain the following:(a) When burning gas is forced downwards out of a rocket motor, the rocket accelerates up.(b) When the water cannons on the boat in the photo are operating, the boat acceleratesforwards.(c) When you step forwards off a skateboard, theskateboard accelerates backwards.(d) A table tennis ball is immersed in a fluid andwaterheld down by a string as shown in Figure 2.56.The container is placed on a balance. What willhappen to the reading of the balance if thestring breaks?stringbalanceFigure 2.5643