You are trying to push your stalled car. Although you apply a horizontal force of 400 N to the car, it doesn’t budge, and neither do you. Which force(s) must also have a magnitude of 400 N?(a) The force exerted by the car on you.(b) The friction force exerted by the car on the road.(c) The normal force exerted by the road on you.(d) The friction force exerted by the road on you.

In this problem, you can determine the magnitude of the force exerted on the car by using the third law of Newton.

Also, in estimating the frictional force, Newton’s second law can be applied.

Given data:

The force exerted by the person on the car is $F=400\mathrm{N}$.

The normal force applied by the road on the person will be equivalent to the weight of the person whose value is not 400 N. Thus, option (c) is the incorrect answer.

The force exerted by the car on the person will be equal to the magnitude of the force the person exerts on the stellar car because of Newton’s third law. The car is not moving, and only horizontal and frictional forces act on the car to push it.

From Newton’s second law, these two forces should have an identical magnitude in the opposite direction. So, due to friction, the road will exert a force of 400 N on the car, and the car will also apply a force of the same magnitude on the road.

The person is not accelerating, and only two forces are acting on him: a horizontal force by the car and a frictional force by the ground. Newton’s second law claims that the ground will exert a similar amount of force of 400 N on the person but in the opposite direction.

Thus, options (a), (b), and (d) are the correct answer.