Exam practice
Every question from the end of every unit, with hints and answers to open. After you check an answer, mark how you did, and come back to the ones you didn’t get.
28 questions
Unit 6 · Force and Motion
2. Fill in the blanks
The distance covered in unit time is ……… .
Answer
speed
The unit of power is ……… .
Answer
watt
The rate of doing work is called ……… .
Answer
power
The use of ball bearings reduces ……… .
Answer
friction. (“Gravity” and “velocity” are not used.)
3. Answer the following questions
Write the SI unit of each of these physical quantities: distance, displacement, work, power, speed, velocity.
Answer
| Quantity | SI unit |
|---|---|
| distance | meter (m) |
| displacement | meter (m) |
| work | joule (J) |
| power | watt (W) |
| speed | meter per second (m/s) |
| velocity | meter per second (m/s) |
Explain distance and displacement with suitable examples.
Answer
Distance is the total length of the path an object travels. It has only magnitude, so it is a scalar. Example: a student walks 500 m along a winding lane from home to school; the distance is 500 m.
Displacement is the shortest, straight-line distance from the starting point to the end point, in a particular direction. It has magnitude and direction, so it is a vector. Example: if the school is 300 m east of the home in a straight line, the displacement is 300 m east.
After a round trip (home to school and back), the distance is 1000 m but the displacement is zero.
“Friction is a necessary evil.” Justify the statement.
Answer
Friction is necessary because we can’t do without it: it lets us walk without slipping, lets tyres grip the road, lets brakes stop vehicles, holds nails and screws in place, and lets us hold things and write.
It is an evil because it also causes harm: it turns useful energy into wasted heat, lowers the efficiency of machines, and wears out machine parts, tyres and shoes.
Since it is both useful and harmful, friction is called a necessary evil.
Define work and write its unit.
Answer
Work is said to be done when a force moves an object in the direction of the force. It is measured as the product of the force and the displacement in the direction of the force: W = F × d. Its SI unit is the joule (J); 1 J of work is done when a force of 1 N moves an object 1 m in its direction.
Write any three ways of reducing friction.
Answer
- Using lubricants such as oil, grease and mobil between moving parts.
- Using wheels and ball bearings, because rolling friction is less than sliding friction.
- Polishing surfaces to make them smooth.
- Making fast-moving objects (airplanes, boats) streamlined.
(Any three.)
Define power and give its unit.
Answer
The work done in unit time by a person or a machine is called its power: power = work ÷ time. It is the rate of doing work. Its SI unit is the watt (W); 1 W = 1 J/s.
Complete the formulas: work = ……, speed = ……, velocity = ……, power = …… .
Answer
work = force × displacement
speed = distance ÷ time
velocity = displacement ÷ time
power = work ÷ time
“Friction also helps us.” Justify the statement.
Answer
Many everyday jobs are possible only because of friction:
- We can walk without slipping because of friction between our feet and the ground.
- Vehicles move and stop because of friction between the tyres and the road, and between the brake pads and the wheels.
- Nails, screws, nuts and bolts hold things together because of friction.
- We can hold a pen and write, and tie knots, because of friction.
So friction is helpful in our daily life.
Why do we reduce friction?
Answer
Friction in machines turns useful energy into wasted heat, so the machine needs more effort and its efficiency goes down. Friction also wears out the moving parts, so the machine breaks down sooner. Reducing friction makes machines work more efficiently, use less fuel or electricity, and last longer.
A car starts from A and goes to D through B and C, at a steady speed of 5 m/s, as in Figure 6.13. A to B is towards the east, B to C towards the south, and C to D towards the west.
- i.What is the velocity of the car on the path AB? Why?
- ii.What is its velocity on BC? Why?
- iii.What is its velocity on CD? Why?
Answer
- i.5 m/s towards the east. On AB the car moves in a straight line towards the east at 5 m/s.
- ii.5 m/s towards the south. The speed is the same, but the direction has changed.
- iii.5 m/s towards the west.
The car’s speed is 5 m/s all the time, but its velocity is different on each part, because velocity includes the direction, and the direction changes at every corner.
Is a person pushing the wall of a house doing work? Give a reason.
Answer
No. Work is done only when the force moves the object in the direction of the force. The wall doesn’t move (its displacement is zero), so the work done on it is F × 0 = 0, however hard the person pushes.
Two people do the same amount of work. When is their power the same, and when is it different? Explain.
Answer
Power = work ÷ time. If both do the same work in the same time, their power is the same. If one takes less time than the other for the same work, that person has more power. For example, if Dataram digs a field in 3 hours and Sohan digs the same field in 2 hours, Sohan’s power is greater.
A person walks along a level road carrying a load on his head. Is he doing work on the load? Explain.
Answer
No work is done against gravity on the load. Gravity pulls the load downwards, and his head holds it up with an upward force, but the load moves sideways, not up. The force and the motion are not in the same direction, so in science no work is done on the load, even though the person gets tired (his muscles use energy just to hold it). He would do work against gravity on the load only if he lifted it, or climbed uphill with it.
4. Solve these numerical problems
Rohit runs 400 m along a straight path in 35 s. Calculate his velocity.
Answer
Displacement (s) = 400 m (straight path, in one direction), time (t) = 35 s
velocity = s ÷ t = 400 m ÷ 35 s = 11.43 m/s, in the direction he runs.
An object covers a distance of 100 m in 5 s. Calculate its speed.
Answer
speed = d ÷ t = 100 m ÷ 5 s = 20 m/s
What distance does a car cover in 10 s, if it is traveling at a speed of 20 m/s?
Answer
distance = speed × time = 20 m/s × 10 s = 200 m
A crane takes 20 s to lift a jeep of mass 1500 kg to a height of 120 m. Calculate the work done by the crane and its power. (g = 9.8 m/s²)
Answer
Here, m = 1500 kg, h = 120 m, t = 20 s, g = 9.8 m/s²
Force needed = weight = m × g = 1500 × 9.8 = 14,700 N
Work = F × h = 14,700 N × 120 m = 1,764,000 J
Power = W ÷ t = 1,764,000 J ÷ 20 s = 88,200 W (88.2 kW)
A porter lifts a 50 kg sack of rice from the ground onto the roof of a bus, 3 m high, as in Figure 6.12. Then he slides the sack 4 m towards the front of the bus, pushing with a force of 450 N. (g = 9.8 m/s²)
- i.How much work does he do against gravity?
- ii.How much work does he do against friction?
Answer
i. Force needed = weight = m × g = 50 × 9.8 = 490 N
Work against gravity = weight × height = 490 N × 3 m = 1470 J
ii. Work against friction = force × distance = 450 N × 4 m = 1800 J
An object moves along the path ABCD, as in Figure 6.14. It goes from A to B at 4 m/s for 4 s, from B to C at 3 m/s for 2 s, and from C to D at 2 m/s for 6 s. What is the total distance it covers?
Answer
distance = speed × time for each part:
AB = 4 m/s × 4 s = 16 m
BC = 3 m/s × 2 s = 6 m
CD = 2 m/s × 6 s = 12 m
Total distance = 16 m + 6 m + 12 m = 34 m
5. Write the differences
Distance and displacement.
Answer
| Distance | Displacement |
|---|---|
| The total length of the path traveled. | The shortest (straight-line) distance from the start to the end, in a particular direction. |
| It has magnitude only: a scalar. | It has magnitude and direction: a vector. |
| distance = speed × time | displacement = velocity × time |
| It can’t be zero once the object has moved. | It is zero after a round trip. |
Speed and velocity.
Answer
| Speed | Velocity |
|---|---|
| The distance covered in unit time. | The displacement in unit time. |
| It has magnitude only: a scalar. | It has magnitude and direction: a vector. |
| speed = distance ÷ time | velocity = displacement ÷ time |
| Example: 5 m/s. | Example: 5 m/s towards the east. |
Work and power.
Answer
| Work | Power |
|---|---|
| Work is done when a force moves an object in the direction of the force. | Power is the work done in unit time, the rate of doing work. |
| work = force × displacement | power = work ÷ time |
| It doesn’t depend on how long the job takes. | It depends on the time: the faster the job, the more the power. |
| SI unit: joule (J). | SI unit: watt (W). |
6. Give reasons
It is harder to drag an object over a rough surface than over a smooth one.
Answer
A rough surface has bigger bumps and dips. As the object is dragged, the bumps of the two surfaces catch on each other more, so the friction is greater, and a bigger force is needed to keep the object moving. A smooth surface has smaller bumps, so the friction is less.
A person standing still while holding up a heavy load is not doing work.
Answer
Work is done only when a force moves an object in the direction of the force. The person pushes the load up, but the load doesn’t move at all, so its displacement is zero and the work done on it is force × 0 = 0. The person gets tired because the muscles use energy to hold the load, but in science no work is done on it.