The lawnmower accelerates in the positive horizontal direction, so that the net horizontal force is, by Newton's second law,
(70 N) cos(-50°) = <em>m</em> (1.8 m/s²)
where <em>m</em> is the mass of the lawnmower. Solve for <em>m</em> :
<em>m</em> = ((70 N) cos(-50°)) / (1.8 m/s²)
<em>m</em> ≈ 25 kg
The lawnmower presumably doesn't get lifted off the ground, so that the net vertical force is 0. If <em>n</em> is the magnitude of the normal force, then by Newton's second law,
<em>n</em> - <em>m g</em> + (70 N) sin(-50°) = 0
<em>n</em> = <em>m g</em> + (70 N) sin(50°)
<em>n</em> = (25 kg) (9.8 m/s²) + (70 N) sin(50°)
<em>n</em> ≈ 300 N
