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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 9

Lesson 1: Introduction to forces and free body diagrams

# Types of forces and free body diagrams

Sal defines and compares tension, weight, friction and normal forces using free body diagrams.

## Want to join the conversation?

• Why is the 5 N force pushing down on the 10 N object a normal force? • If a girl who weighs 40 kg was to climb up a 20-foot tree at a constant acceleration, what would be the work done by the girl?

The formula for work is W=Force x Displacement
Newton's second law says F=ma, so if a = 0, wouldn't the force be zero and the work be zero. This doesn't make sense though because there has to be some work done. • At in the free body diagram at the top left of the screen, why is the normal force 5N upward. I understand why it is upward, but why is it 5N. Should it not be 10N because the shelf is 10N? • At i don't understand why the normal force is pushing downwards, i thought normal force reacts in the opposite direction of gravity. • No, not always. The normal force is (you guessed it) normal to the surface the object is in contact with. If you hold it against the ceiling, the normal force is in the same direction as gravity. If you hold it against the wall, the normal force is perpendicular to force of gravity. There is no law which states that the normal force must be in the opposite direction as gravity. Hope this helps!
• In the last example with the shelf and the block of the shelf, why is the normal force acting downward. I would think that gravity is acting downward on the shelf but that force is counteracted by the tension force in the ropes holding the shelf(this is without the block on the shelf). When the block is on the shelf though, gravity is acting downward on the block but the force of gravity is counteracted by the normal force from the shelf keeping it from accelerating downwards. • Is friction related to inertia? • Yes, it is, in fact, it can be due to friction an object has inertia. Inertia is defined as an object will be in motion as long as it is affected by an external force(usually friction, but not always). An example of friction being related to inertia: A book is sliding across a table and it stops after a certain amount of time. The book stops because of the frictional force that is affecting the book while it is sliding across the table. While that answers your question I also want to give an example of when friction is not necessarily related to inertia. For example, you are driving a car and suddenly you see a deer in front of you and you slam your brakes, meanwhile, your book is in the back seat and it flies and hits the back of your seat. The external force here is your seat that stops the book to keep flying. Friction may play a small role, but it did not play a major role relating to the inertia of the object. Sorry for a long response, but if you have any questions please do reply back. Just a clarification, I am taking my first year of Physics so if I make a mistake or need to add anything please suggest it. Thank you.
• what happens to newton if you let go of an untied balloon? • If I am holding a book in my hand would there be a normal force or are normal forces only for surfaces such as tables, floors, etc..? • Sal tells us that when the box falls onto the table or the Earth, the force of gravity pulling the box to the Earth and the force exerted by the table on the box cancel out, keeping the box from phasing through the table. However, this makes little sense because when forces cancel out then there should be no change in an object's velocity, so what actually stops the box from moving/phasing through everything? • On impact, the normal force exerts an additional impulse force on the box (and the table). The impulse force is responsible for the change in momentum of the falling object.

Because of Newton’s 3rd law, the force would act on both the object falling (the box) and the table. Sometimes the table is not able to overcome the stress caused by this additional force, so the table breaks and the object can continue it’s motion downward(probably damaged itself as well), with the amount of its original momentum lost being dependent on the amount of time spent impacting the table.

That is why dropping a box onto a table from a tall height can cause more damage to both the table and the box, than lightly resting the box on the table instead. The impulsive forces in these scenarios are different.
(1 vote)
• At the 5 minute mark, why is it friction that is the cause of the object not moving, couldn't it be mass?
(1 vote) • Mass triggers friction (if on Earth or other body), and its the friction that causes the object to move:

At the 5 minute mark, the friction from all the previous moments already reduced the velocity of the object. It is at the 5th minute when it stopped moving.

Let's see it this way (let's say it looses 1 m/s each minute):

When launched:5 m/s
1st minute: 4 m/s
2nd minute: 3 m/s
3rd minute: 2 m/s
4th minute: 1 m/s
5 minute: 0 m/s

So at the 4.9th minute, its velocity is probably close to 0 but not exactly 0 (0.1 m/s). At the 5th moment, its 0. It stops.

In this case, if the example of Earth, its the mass of the object which causes the gravitional force between it and the Earth, triggering the friction to slow it down.