Question
Question: What normal force is exerted on a stationary flat plate held perpendicular to a jet of water? The ho...
What normal force is exerted on a stationary flat plate held perpendicular to a jet of water? The horizontal speed of the water is 80cm/s and 30cm3 of the water hits the plate each second. Assume that the water moves parallel to the plate after striking it. One cubic centimeter of water has a mass of one gram.
A. 0.03N
B. 0.024N
C. 0.04N
D. 0.036N
Solution
We have learned that the net force acting on an object is equal to the change in momentum over time. This law can be applied here. With the given speed and the volume of water we can find out momentum of water. Substitute the value of momentum in the force - momentum equation to find out the force acting on the plate.
Formula used:
Momentum = mass × velocity
Force = time intervalFinal momentum - initial momentum
Complete answer:
Given,
Speed of water = 80cm/s = 0.8m/s
Volume of water hitting the plate = 30cm3
Mass of 1cm3water = 1g
Then,
Mass of 30cm3water = 1×30 = 30g = 0.03Kg
We have,
Momentum = mass × velocity
Here,
Velocity of water is 0.8m/s
Then,
Momentum of 0.03Kg water hitting the plate = 0.03×0.8 = 0.024 Kgm/s
Hence, Initial momentum = 0.024 Kgm/s
We have,
Force = time intervalFinal momentum - initial momentum
Velocity of water after hitting the plate is zero as the water moves parallel to the plate after striking it.
Then,
Final momentum of water = 0 Kgm/s And, time =1s
Therefore,
Force on plate = 10.024-0=0.024N
Normal force on plate, N= 0.024N
So, the correct answer is “Option B”.
Additional Information:
A normal force is a contact force which surfaces exert to prevent objects from passing through each other. It is not applicable for two surfaces which are not in contact. It acts perpendicular to the surface. If normal force is the only force counteracting on a body, it is simply equal to the weight of the object. It is represented as N. The normal force can be less than the weight of the object, if the object is on an inclined surface.
Note:
A normal force acts on a body even if it is at rest. The magnitude of the normal force can only be determined by analyzing the situation. The normal force acting on a body changes in response to other forces acting on the object. And normal force can never be zero.