1.when a train pulls a wagon, is the work zero because wagon pulls on the train with equal opposite force(newton's third law)?

Question

2.what is negative net work and when it is done an on object, does it reduce objects kinetic energy?
3.if the work done on an object is zero, does either the force or the displacement HAVE to be zero in magnitude?

Answer 1

1. No The train does work; the wagon has work done on it.
2, Negative net work ON an object is work done BY that object against another force. When negative work is done ON an object, its kinetic energy may decrease, but not necessarily. The work done can be converted to other forms of energy.
3. No. They can be in perpendicular directions

Answer 2

Thanks one more question though.

When two identical masses are placed on frictionless slopes at identical heights. The first slope makes an angle of 30deg with respect to horizontal, second one with 40deg. Both balls are released simultananeously. At the bottom of the slope would the two balls have same speed because gravitational force is conserved?

Answer 3

Yes, in a frictionless situation such as you have described