Class -6 Physics Chapter3 - Force
Short-Answer Questions
Q1. What is force? Write the SI unit of force.?
Ans: Force is something that can bring about a change in the state of rest or of motion of a body , or a change in the shape or size of a body. The S.I unit of force is newton ( N)
Q2.
Ans: Forces that two bodies in contact apply on each other are called contact forces. Friction and Tension are examples of a contact forces.
Q3. Mention a force that is not a contact force.
Ans:- Gravitational force is not a contact force.
Q4. What is friction? How is the nature of the surfaces in contact related to the force of friction?
Ans: The force that opposes or prevents motion between two surfaces in contact is called the force of Friction.
Friction is large for rough surfaces and small for smooth surfaces.
Q5. Why is it difficult to write on glass with a pencil?
Ans: Pencils write because of friction.It is difficult to write on glass with a pencil because of less friction between them
Q6.
Ans: The maximum value of static friction is called Limiting friction.
Q7.
Ans: Three methods of reducing friction in a machine are
1. Using Lubricants
2.Ussing ball bearings
3. By making machine parts smooth
Q8.
Ans: Ball bearings are used to reduce friction in machines .
Q 9.
Ans: The heat produced in friction is used to light matches.
Long-Answer Questions:
Q1.
Ans: The effects produced by a Force -
1. Force can cause motion
Example- A child is pushing a toy car and the car moving in the direction of force applied.
2 Force can increase speed
Example -If a ball is kicked in the direction it is rolling, its speed increases.
3.Force can decrease speed
Example - Brakes push against wheels to stop their motion.
4. Force can change the direction of motion.
Example - A ball bounces back when hits on the floor.
Q2.
Ans: Two examples in which friction is of advantage to us-
1. Brakes are applied to stop a vehicle. The force of friction between the brakeshoes and the wheels slows down the wheels.
2. Friction allows us to grip an object (for example, a pencil) between our fingers. The object has a tendency to fall. The force of friction opposes this tendency.
Ans: Disadvantages of friction-
1. Friction opposes motion
2. Friction produces heat
3. Friction causes wear and tear
(b) A knife becomes blunt with use because Frictional force acts between the knife and the object. After continuous use wear occurs on the surface of the knife in contact and so over time, it becomes blunt.
Q4.
Ans: . Static friction acts as long as the two surfaces in contact are at rest. Kinetic or sliding friction comes into play when sliding begins. Static is greater than kinetic friction.
Choose the correct option:
1. . Which of the following increases friction?
2. In which of the following cases is friction a disadvantage to us?
(a) Running
(b) Braking
(c) Writing with a pencil
(d) Parts of a machine rubbing against each other
3. Which of the following is the least? (a) Static friction
4. Which of the following is the greatest?
Fill in the blanks:
1. The tension in a string is an example of contact force.
2. The force of friction is less on wet surfaces.
3. When you walk or run, you push your foot backward
4. Friction is large for rough surfaces and small for smooth surfaces .
5. Friction is much less in rolling than in sliding .
6. Materials used to reduce friction are called Lubricants .
Write true or false:
1. Friction acts only when two surfaces move over each other. - False
2. Rolling friction is less than sliding friction.- True
3. No contact force acts on a bucket when it is being pulled out of a well using a rope.- False
4. The force of friction always opposes motion or the tendency of motion.- True
5. All forces are contact forces.- False
6. A boat is streamlined to reduce the resistance to motion offered by water.- True
It is very useful
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