Work and Energy Worksheet-4
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A car weighing 1000 kg and traveling at 30 m/s stops at a distance of 50 m decelerating uniformly. What is the force exerted on it by the brakes? What is the work done by the brakes?
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How much work is done when a body of mass m is raised to a height h above the ground?
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State the SI unit of work.
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Is work a scalar or a vector quantity?
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Define 1 joule of work.
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What is the condition for a force to do work on a body?
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Is energy a vector quantity?
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What are the units of (a) work, and (b) energy?
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What is the work done against gravity when a body is moved horizontally along a frictionless surface?
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By how much will the kinetic energy of a body increase if its speed is doubled?
Answer:
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In order to calculate the force, we have to find out the acceleration or retardation.
Initial speed, u = 30 m/s
Final speed, v = 0 (Since the car stops)
Acceleration a = ?
Distance s = 50 m
Now, we know that:
v2 = u2 + 2as
So, (0)2 = (30)2 + 2 × a × 50
100 a = –900
a =–(900/100)
Acceleration, a = –9 m/s2
The force exerted by the brakes can now be calculated by using the formula :
F = m × a
Here, Mass, m = 1000 kg (Given)
Acceleration, a = –9 m/s2
Force, F = 1000 × (–9)
F = –9000 N
Thus, the force exerted by the brakes on the car is of 9000 newtons. The negative sign shows that it is a retarding force.
The work done by the brakes :
W = F × s
Here, Force, F = 9000 N
Distance, s = 50 m
So, Work done, W = 9000 × 50 J
= 450000 J
= 4.5 × 105 J
Thus, the work done by the brakes is 4.5 × 105 joules.
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m × g × h
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Joule
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vector
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When a force of 1 Newton moves a body through a distance of 1 meter in its own direction, then the work done is known as 1 joule. This is
1 joule = 1 Newton ´ 1 meter
Or 1 J = 1 Nm
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The force should produce motion in the body.
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No it is a scalar quantity
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Joule
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Zero
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Becomes four times
