Most people like bikes. Perhaps you ride a bike to school. Riding a bike can be great of fun.Do you know how to ride a bike safely?

    Careful boys and girls learn to ride their bikes well before they go out on the road.How can you tell whether you are a good rider?

    If you can ride a bike in a straight line,you can ride well .If you can't ,you should practice riding a in a safe place.

    To be safe on a bike, you need to use both hands and both feet. Keep your feet on the pedals(脚蹬)and your hands on the handle bars.

    You know that your bike is only for one person. It's not safe to carry anyone with you. Keep yourself safe when you ride on a bike.Watch where you are going all times.

如图，轻绳OA一端系在天花板上，与竖直方向夹角37°．轻绳OB水平，一端系在墙上，结点O处挂一重为40N的物体．