# F2 Forces – Fill in the blanks

ID# 1948
 1 The unit of force is the newton【牛頓】 and it can be measured by a newton balance【牛頓秤】 or a spring balance【弹簧秤】. A force can change the speed, the direction of motion and the shape of an object. Catching a football changes it’s speed. Striking a volleyball changes its speed and direction of motion. Pressing on a inflated balloon【充氣氣球】 changes its shape. 2 When an object applies【施加】 a force on another objects through direct contact, the force is called a contact force while an object applies a force on another object that is not in direct contact, the force is called a non-contact force. 3 Place two bar magnets on a bench with opposite poles【相反的兩極】 facing each other, then move one magnet slowly towards the other. We will see the magnets attract【吸引】 each other. 4 Place two bar magnets on a bench with same poles【相同的兩極】 facing each other, then move one magnet slowly towards the other. We will see the magnets repel【排斥】 each other. 5 Magnetic force and the force of gravity【重力】 are examples of non-contact force. 6 When an object slides【滑動】 over another object, a force arises【出現】 to oppose【抵抗】 this change. We call this force friction【摩擦力】. 7 Friction is a contact force between two contacting surfaces【接觸表面】 that slide【滑動】 on each other. It always opposes【抵抗】 the motion of an object. 8 The faster an object moves in water or air, the greater the opposing force acting on it. The opposing force acting on an object by air is called the air resistance【空氣阻力】. 9 When an object moves against friction【抵抗摩擦】, its kinetic energy changes to heat energy. This energy conversion is often unnecessary and leads to【引致】 a waste of energy. Therefore, different ways have been found to reduce friction to save energy. 10 For an object moving in air or water, the resistance【阻力】 can be reduced【降低】 by making the object streamlined【流線型】 in shape. 11 The weight of an object on Earth is the force of gravity【重力】 acting on the object by the Earth. It is measured in newtons. The mass of an object shows the amount of matter【物質量】 in the object. It is measured in kilograms or grams.