Affect of Friction on Acceleration and Deceleration

[Molly1235]

Homework Statement

"Explain why friction is important in accelerating and decelerating a car. In your answer discuss factors affecting the magnitude of the acceleration and state the direction in which friction acts for both acceleration and deceleration.

Homework Equations

None, as far as I know. No calculations required.

The Attempt at a Solution

I'm really really stuck. All I could think of was when you drive a car on ice there is NO friction, meaning that if you try to accelerate or decelerate there is no gradual change in velocity - it just keeps going. But I don't know how to relate that to the question. Would be really grateful for some help!

Thank you

 

[I like Serena]

Hi Molly!

You already got it.
With no friction there is no acceleration and no deceleration.

When there is friction, of course you do get acceleration.
The thing you probably still need, is that with friction and acceleration, the car is accelerated forward, meaning there is a net resultant forward force on the car.

Since Newton mentioned that action is equal to the opposite of reaction, that means there is an equal and opposite force exerted on the ground.
When the car accelerates, a forward force acts on the car, while at the same time the ground is pushed backward.
With deceleration it works the same way but with opposite forces.

 

[Molly1235]

Hiya!

Thank you for your reply - I'm afraid I'm still a little confused as to how a car can accelerate when friction is still acting upon it? If every force is met with an equal and opposite force, how does the object still move? (I know this is a stupid question but I've had a complete blank).

 

[I like Serena]

The friction between the car and the ground is essential.
But it has 2 components.
One component is the car acting on the ground, pushing it backward.
Without friction the wheels would just spin, but with traction the ground is pushed backward.
The other component is the ground acting on the car.
When the car pushes the ground back, the ground pushes back (equal but opposite), accelerating the car.

 

[LeonhardEu]

The real point is that action and reaction are exerted on different bodies. when the car exerts force ( through friction between the tires and the ground) TO the ground gets the reaction FROM the ground to the opposite direction. The car pushes back the ground so is pushed forward by the ground and vice versa.
Without friction there is no force.

 

[Molly1235]

Aaah thank you! I understand now - you've both been very helpful!