Question

A shopper pushes a grocery cart 20m at constant speed on level ground against a 35N frictional force. He pushes in a direction 25 degrees below the horizontal. What is the work done on the cart by gravity?

I know you can use W=Fdcos(theta) where F=mass*gravity. BUT, no mass is given so there must be some other way.

Help!

Answer 1

gravity is doing no work because its perpendicular to the motion

Answer 2

mass is not required. Constant speed means zero acceleration.
Fcos(theta) - Ff=m*a
Fcos(theta) - Ff=0
F cos(25) =35
0.9063F=35
F=38.62N
W=Fcos(theta)*d
W= 38.62 cos(25)*20
W=700J