力学Forces and their Effects.pdf

? Boardworks Ltd 20041 of 20 67 KS3 Physics 7K Forces and their Effects ? Boardworks Ltd 20041 of 20 62 7 7K Forces and their Effects Contents Feel the force Weight and mass Forces in water Friction Summary activities ? Boardworks Ltd 20041 of 20 63 7 A force is a push, pull or twist. A force cannot be seen but you can see how a force affects an object. What is a force? ? Boardworks Ltd 20041 of 20 64 7 Forces activity ? Boardworks Ltd 20041 of 20 65 7 10 N 10 N If you link two forcemeters and pull equally hard from both ends, the forces recorded on both will be the same. balanced forces When forces are balanced The forces acting on the central hooks cancel each other out – they are equal in magnitude and opposite in direction. Because the forces are balanced the hooks do not move. ? Boardworks Ltd 20041 of 20 66 7 11 N 10 N What happens if the pull on one end is greater than the pull on the other end? unbalanced forces movement When forces are unbalanced The forces acting on the hooks are no longer balanced. Both hooks will start to move to the left – their speed will change. This is called acceleration. Unbalanced forces lead to a change in speed or direction. ? Boardworks Ltd 20041 of 20 67 7 Balanced or unbalanced forces? ? Boardworks Ltd 20041 of 20 68 7 7K Forces and their Effects Contents Feel the force Weight and mass Forces in water Friction Summary activities ? Boardworks Ltd 20041 of 20 69 7 If you rub your hands together they get warm. There is resistance to the rubbing motion. What is the name of this resistive force called? What causes this force? Your hands might look smooth, but on a microscopic level they have rough surfaces. So when you rub your hands together you feel the resistive force of friction. What is friction? It is called friction. ? Boardworks Ltd 20041 of 20 60 of 27 Friction always tries to slow moving objects down. We say it opposes motion. There is a maximum value for the frictional force which depends on: More ab