2 Mechanics - Pearson Global Schools

Exercises19 A ball of weight 10 N is suspended on a string and pulled to one side by another horizontal stringas shown in Figure 2.36. If the forces are balanced:(a) write an equation for the horizontal components of the forces acting on the ball(b) write an equation for the vertical components of the forces acting on the ball(c) use the second equation to calculate the tension in the upper string, T(d) use your answer to (c) plus the first equation to find the horizontal force F.20 The condition for the forces to be balanced is that the sumNof components of the forces in any two perpendicularcomponents is zero. In the ‘box on a ramp’ example thevertical and horizontal components were taken.However, it is sometimes more convenient to considercomponents parallel and perpendicular to the ramp.Consider the situation in Figure 2.37. If the forces on this30°box are balanced:50 N(a) write an equation for the components of the forcesFigure 2.37parallel to the ramp(b) write an equation for the forces perpendicular to the ramp(c) use your answers to find the friction (F) and normal force (N).21 A rock climber is hanging from a rope attached to the cliff by two bolts asshown in Figure 2.38. If the forces are balanced(a) write an equation for the vertical component of the forces on the knot(b) write an equation for the horizontal forces exerted on the knot(c) calculate the tension T in the ropes joined to the bolts.The result of this calculation shows why ropes should not be connected in this way.F30°TF10 NFigure 2.3680° 80°TTknot600 NFigure 2.382.6 Newton’s laws of motionAssessment statements2.2.4 State Newton’s first law of motion.2.2.5 Describe examples of Newton’s first law.2.2.6 State the condition for translational equilibrium.2.2.7 Solve problems involving translational equilibrium.We now have the quantities to enable us to model motion and we have observedthat to make something start moving we have to exert a force – but we haven’tconnected the two. Newton’s laws of motion connect motion with its cause. Inthis course there are certain fundamental concepts that everything else rests upon,Newton’s laws of motion are among the most important of these.Using laws in physicsA law in physics is a very usefultool. If applied properly, it enablesus to make a very strong argumentthat what we say is true. If asked‘will a box move?’ you can say thatyou think it will and someone elsecould say it won’t. You both haveyour opinions and you would thenargue as to who is right. However,if you say that Newton’s law says itwill move, then you have a muchstronger argument (assuming youhave applied the law correctly).Newton’s first lawA body will remain at rest or moving with constant velocity unless acted uponby an unbalanced force.To put this the other way round, if the forces on a body are unbalanced, then it willnot be at rest or moving with constant velocity. If the velocity is not constant thenit is accelerating.35